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Full text of "The elements of geology"

tEMENTS OF GEOLOGY 



NORTON 



GIFT OF 
Agriculture education 




THE 



ELEMENTS OF GEOLOGY 



BY 
WILLIAM HARMON NORTON 

PBOFESSOK OF GEOLOGY is COBNELL COLLEGE 



GINN AND COMPANY 

BOSTON NEW YORK CHICAGO LONDON 
ATLANTA DALLAS COLUMBUS SAN FRANCISCO 



COPYRIGHT, 1905, 1921, BY 
WILLIAM HARMON NORTON 



ALL RIGHTS RESERVED 

--r 

321.5 

AGRIC. DEFT. 



fltftenatum 



(iINN AND COMPANY PRO- 
PRIETORS BOSTON U.S.A. 



PKEFACE 

Geology is a science of such rapid growth that no apology is 
expected when from time to time a new text-book is added to 
those already in the field. The present work, however, is the 
outcome of the need of a text-book of very simple outline, in 
which causes and their consequences should be knit together as 
closely as possible, a need long felt by the author hi his teach- 
ing, and perhaps by other teachers also. The author has ven- 
tured, therefore, to depart from the common usage which sub- 
divides geology into a number of departments, dynamical, 
structural, physiographic, and historical, and to treat in im- 
mediate connection with each geological process the land forms 
and the rock structures which it has produced. 

It is hoped that the facts of geology and the inferences drawn 
from them have been so presented as to afford an efficient dis- 
cipline in inductive reasoning. Typical examples have been 
used to introduce many topics, and it has been the author's aim 
to give due proportion to both the wide generalizations of our 
science and to the concrete facts on which they rest. 

There have been added a number of practical exercises such 
as the author has used for several years in the class room. 
These are not made so numerous as to displace the problems 
which no doubt many teachers prefer to have their pupils solve 
impromptu during the recitation, but may, it is hoped, suggest 
their use. 

In historical geology a broad view is given of the develop- 
ment of the North American continent and the evolution of 

iii 



iv PREFACE 

life upon the planet. Only the leading types of plants and 
animals are mentioned, and special attention is given to those 
which mark the lines of descent of forms now living. 

By omitting much technical detail of a mineralogical and 
paleontological nature, and by confining the field of view almost 
wholly to our own continent, space has been obtained to give 
to what are deemed for beginners the essentials of the science 
a fuller treatment than perhaps is common. 

It is assumed that field work will be introduced with the 
commencement of the study. The common rocks are therefore 
briefly described in the opening chapters. The drift also receives 
early mention, and teachers in the northern states who begin 
geology in the fall may prefer to take up the chapter on the 
Pleistocene immediately after the chapter on glaciers. 

Simple diagrams have been used freely, not only because they 
are often clearer than any verbal statement, but also because 
they readily lend themselves to reproduction on the blackboard 
by the pupil. The text will suggest others which the pupil may 
invent. It is hoped that the photographic views may also be 
used for exercises in the class room. 

The generous aid of many friends is recognized with special 
pleasure. To Professor W. M. Davis of Harvard University there 
is owing a large obligation for the broad conceptions and lumi- 
nous statements of geologic facts and principles with which he 
has enriched the literature of our science, and for his stimulating 
influence in education. It is hoped that both in subject-matter 
and in method the book itself makes evident this debt. But 
besides a general obligation shared by geologists everywhere, and 
in varying degrees by perhaps all authors of recent American 
text-books in earth science, there is owing a debt direct and 
personal. The plan of the book, with its use of problems and 
treatment of land forms and rock structures in immediate con- 
nection with the processes which produce them, was submitted 



PREFACE V 

to Professor Davis, and, receiving Ms approval, was carried into 
effect, although without the sanction of precedent at the time. 
Professor Davis also kindly consented to read the manuscript 
throughout, and his many helpful criticisms and suggestions are 
acknowledged with sincere gratitude. 

Parts of the manuscript have been reviewed by Dr. Samuel 
Calvin and Dr. Frank M. Wilder of the State University of 
Iowa ; Dr. S. W. Beyer of the Iowa College of Agriculture and 
Mechanic Arts; Dr. U. S. Grant of Northwestern University; 
Professor J. A. Udden of Augustana College, Illinois ; Dr. C. H. 
Gordon of the New Mexico State School of Mines ; Principal 
Maurice Eicker of the High School, Burlington, Iowa ; and the 
following former students of the author who are engaged in the 
earth sciences : Dr. W. C. Alden of the United States Geological 
Survey and the University of Chicago ; Mr. Joseph Sniff en, in- 
structor in the Academy of the University of Chicago, Morgan 
Park ; Professor Martin lorns, Fort Worth University, Texas ; 
Professor A. M. Jayne, Dakota University; Professor G. H. 
Bretnall, Monmouth College, Illinois; Professor Howard E. 
Simpson, Colby College, Maine ; Mr. E. J. Cable, instructor in 
the Iowa State Normal College; Principal C. C. Gray of the 
High School, Fargo, North Dakota ; and Mr. Charles Persons of 
the High School, Hannibal, Missouri. A large number of the 
diagrams of the book were drawn by Mr. W. W. White of the 
Art School of Cornell College. To all these friends, and to 
the many who have kindly supplied the illustrations of the text, 
whose names are mentioned in an appended list, the writer 
returns his heartfelt thanks. 

WILLIAM HARMON NORTON 

CORNELL COLLEGE, MOUNT VERNON, IOWA 
July, 1906 



INTRODUCTORY NOTE 

During the preparation of this book Professor Norton has frequently 
discussed its plan with me by correspondence, and we have considered 
together the matters of scope, arrangement, and presentation. 

As to scope, the needs of the young student and not of the expert 
have been our guide ; the book is therefore a text-book, not a reference 
volume. 

In arrangement, the twofold division of the subject was chosen be- 
cause of its simplicity and effectiveness. The principles of physical 
geology come first ; the several chapters are arranged in what is believed 
to be a natural order, appropriate to the greatest part of our country, 
so that from a simple beginning a logical sequence of topics leads 
through the whole subject. The historical view of the science comes 
second, with many specific illustrations of the physical processes pre- 
viously studied, but now set forth as part of the story of the earth, 
with its many changes of aspect and its succession of inhabitants. 
Special attention is here given to North America, and care is taken 
to avoid overloading with details. 

With respect to method of presentation, it must not be forgotten 
that the text-book is only one factor in good teaching, and that in 
geology, as in other sciences, the teacher, the laboratory, and the local 
field are other factors, each of which should play an appropriate part. 
The text suggests observational methods, but it cannot replace observa- 
tion in field or laboratory ; it offers certain exercises, but space cannot 
be taken to make it a laboratory manual as well as a book for study ; it 
explains many problems, but its statements are necessarily more terse 
than the illustrative descriptions that a good and experienced teacher 
should supply. Frequent use is made of induction and inference in 
order that the student may come to see how reasonable a science is 
geology, and that he may avoid the too common error of thinking that 
the opinions of "authorities" are reached by a private road that is 
closed to him. The further extension of this method of presentation 
is urged upon the teacher, so that the young geologist may always learn 
the evidence that leads to a conclusion, and not only the conclusion itself. 

HARVARD UNIVERSITY, CAMBRIDGE, MASS. ^' 

July, 1905 



ACKNOWLEDGMENT OF ILLUSTRATIONS 

Adams, Professor F. D., McGill University, Canada, 241. 

Alden, Dr. W. C., Washington, D.C., 353. 

American Museum of Natural History, New York, 344. 

Ash, H. C., Galesburg, 111., 133. 

Beyer, Dr. S. W., Iowa College of Agriculture, 363. 

Calvin, Dr. Samuel, Iowa State University, 45, 295, 317, 325, 371. 

Carney, Frank, Ithaca, N.Y., 356. 

Clark, Dr. Wm. B., Maryland Geological Survey, 43. 

Borne, Dr. Georg v. d., Jena, Germany, 5, 6. 

Daly, Dr. R. A., Ottawa, Canada, 164. 

Defieux, C. A., Liverpool, England, 154. 

* Detroit Photographic Co., 235, 236. 

* Ellis, W. M., Edna, Kan., 13. 

Fairchild, Professor H. L., University of Rochester, 141, 357. 

Field Columbian Museum, Chicago, 87. 

Forster, Dr. A. E., University of Vienna, 32. 

Gardner, J. L., Boston, 12, 140, 352. 

Geological Survey of Canada, 256. 

Gilbert, Dr. G. K., by courtesy of the American Book Company, 39. 

* Haines, Ben, New Albany, Ind., 33. 

* Haynes, F. J., St. Paul, Minn., 52, 95, 233. 
Henderson, Judge Julius, Boulder, Col., 94. 

James, George Wharton, Pasadena, Cal., 16, 127, 215, 229. 
Johnston-Lavis, Professor H. J., Beaulieu, France, 216. 
King, J. Harding, Stourbridge, England, 119. 
Lawson, Dr. Andrew C., University of California, 113. 
Le Conte, Professor J. N., University of California, 8. 
Libbey, Dr. William, Princeton University, 92. 

* McAllister, T. H., New York, 242. 

* Meyers, H. C., Boise, Id., 19. 

Mills, Professor H. A., Cornell College, 208, 304. 

vii 



viii ACKNOWLEDGMENT OF ILLUSTRATIONS 

Norton, Professor W. H., Cornell College, 14, 35, 59, 88, 128, 183, 226, 
234, 255, 349, 364, 367. 

* Notman, Wm. & Son, Montreal, Canada, 98, 181. 
Obrutschew, Dr. W., Tomsk Technological Institute, Siberia, 73. 
Oldham, Dr. R. D., Geological Survey of India, 120. 

* Peabody, H. C., Pasadena, Cal., 54. 

* Pierce, C. C. & Co., Los Angeles, Cal., 15. 
Pillsbury, Arthur, San Francisco, Cal., 115. 

* Rau, Wm., Philadelphia, 18, 21, 122, 123, 218. 
Reusch, Dr. Hans, Geological Survey of Norway, 112. 

Reynolds, Professor S. H., University College, Bristol, England, 202. 
Ricker, Principal Maurice, Burlington, Iowa, 48, 89. 

* Shepard, E. A., Minneapolis, Minn., 105. 
Smith, W. S. Tangier, Los Gatos, Cal., 186. 

* Soule Photographic Co., Boston, 131. 

U. S. Geological Survey, 3, 4, 23, 25, 34, 41, 63, 69, 78, 79, 80, 110, 
111, 114, 125, 126, 129, 130, 142, 151, 153, 169, 172, 177, 178, 
188, 211, 212, 214, 228, 237, 238, 239, 243, 244, 254, 257, 340, 
341, 353, 355. 

U. S. National Museum, 149, 220, 221, 222, 225, 332. 

* Valentine & Sons, Dundee, Scotland, 40, 136, 227. 
Vroman, A. C., Pasadena, Cal., 17. 

Ward's Natural Science Establishment, Rochester, N.Y., 152. 

* Welch, R., Belfast, Ireland, 1, 37. 

Westgate, Dr. L. G., Ohio Wesleyan University, 66. 
Whymper, Edward, London, England, 106. 
Wilcox, W. D., Washington, D.C., 20. 
Wilson, Dr. A. W. G., McGill University, Canada, 68. 

* Wilson, G. W., & Co., Aberdeen, Scotland, 82, 213. 

* Worsley-Benisori, F. H., Cheapstow, England, 170. 

* Dealer in photographs or lantern slides. 



CONTENTS 



PAGE 

INTRODUCTION THE SCOPE AND AIM OF GEOLOGY ... 1 

PART I 

EXTERNAL GEOLOGICAL AGENCIES 
CHAPTER 

I. THE WORK OF THE WEATHER 5 

II. THE WORK OF GROUND WATER 39 

III. RIVERS AND VALLEYS 54 

IV. RIVER DEPOSITS 93 

V. THE WORK OF GLACIERS 113 

VI. THE WORK OF THE WIND 144 

VII. THE SEA AND ITS SHORES 155 

VIII. OFFSHORE AND DEEP-SEA DEPOSITS 174 

PART II 
INTERNAL GEOLOGICAL AGENCIES 

IX. MOVEMENTS OF THE EARTH'S CRUST 195 

X. EARTHQUAKES 233 

XI. VOLCANOES 238 

XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN . . 265 

XIII. METAMORPHISM AND MINERAL VEINS . 281 



x CONTENTS 

PART III 

HISTORICAL GEOLOGY 

CHAPTER PAGE 

XIV. THE GEOLOGICAL RECORD 291 

XV. THE PRE-CAMBRIAN SYSTEMS 304 

XVI. THE CAMBRIAN 315 

XVII. THE ORDOVICIAN AND SILURIAN 327 

XVIIT. THE DEVONIAN 341 

XIX. THE CARBONIFEROUS 350 

XX. THE MESOZOIC 368 

XXI. THE TERTIARY 394 

XXII. THE QUATERNARY 416 

INDEX 453 



. THE 

ELEMENTS OF GEOLOGY 

INTRODUCTION 
THE SCOPE AND AIM OF GEOLOGY 

Geology deals with the rocks of the earth's crust. It learns 
from their composition and structure how the rocks were made 
and how they have been modified. It ascertains how they have 
been brought to their present places and wrought to their vari- 
ous topographic forms, such as hills and valleys, plains and 
mountains. It studies the vestiges which the rocks preserve of 
ancient organisms which once inhabited our planet. Geology is 
the history of the earth and its inhabitants, as read in the rocks 
of the earth's crust. 

To obtain a general idea of the nature and method of our 
science before beginning its study in detail, we may visit some 
valley, such as that illustrated in the frontispiece, on whose 
sides are rocky ledges. Here the rocks lie in horizontal layers. 
Although only their edges are exposed, we may infer that these 
layers run into the upland on either side and underlie the entire 
district; they are part of the foundation of solid rock which 
everywhere is found beneath the loose materials of the surface. 

The ledges of the valley of our illustration are of sandstone. 
Looking closely at the rock we see that it is composed of 
myriads of grains of sand cemented together. These grains 
have been worn and rounded. They are sorted also, those of 
each layer being about of a size. By some means they have 
been brought hither from some more ancient source. Surely 

1 



:;": THE ELEMENTS OF GEOLOGY 

these grains have had a history before they here found a resting 
place, a history which we are to learn to read. 

The successive layers of the rock suggest that they were 
built one after another from the bottom upward. We may be as 
sure that each layer was formed before those above it as that 
the bottom courses of stone in a wall were laid before the courses 
which rest upon them. 

We have no reason to believe that the lowest layers which 
we see here were the earliest ever formed. Indeed, some deep 
boring in the vicinity may prove that the ledges rest upon other 
layers of rock which extend downward for many hundreds of 
feet below the valley floor. Nor may we conclude that the 
highest layers here were the latest ever laid ; for elsewhere we 
may find still later layers lying upon them. 

A short search may find in the rock relics of animals, such 
as the imprints of shells, which lived when it was deposited; 
and as these are of kinds whose nearest living relatives now 
have their home in the sea, we infer that it was on the flat sea 
floor that the sandstone was laid. Its present position hundreds 
of feet above sea level proves that it has since emerged to form 
part of the land ; while the flatness of the beds shows that the 
movement was so uniform and gentle as not to break or strongly 
bend them from their original attitude. 

The surface of some of these layers is ripple-marked. Hence 
the sand must once have been as loose as that of shallow sea 
bottoms and sea beaches to-day, which is thrown into similar 
ripples by movements of the water. In some way the grains 
have since become cemented into firm rock. 

Note that the layers on one side of the valley agree with 
those on the other, each matching the one opposite at the same 
level. Once they were continuous across the valley. Where the 
valley now is was once a continuous upland built of horizontal 
layers ; the layers now show their edges, or outcrop, on the valley 
sides because they have been cut by the valley trench. 



THE SCOPE AND AIM OF GEOLOGY 3 

The rock of the ledges is crumbling away. At the foot of 
each step of rock lie fragments which have fallen. Thus the 
valley is slowly widening. It has been narrower in the past ; it 
will be wider in the future. 

Through the valley runs a stream. The waters of rains which 
have fallen on the upper parts of the stream's basin are now on 
their way to the river and the sea. Kock fragments and grains 
of sand creeping down the valley slopes come within reach of 
the stream and are washed along by the running water. Here 
and there they lodge for a time in banks of sand and gravel, 
but sooner or later they are taken up again and carried on. The 
grains of sand which were brought from some ancient source to 
form these rocks are on their way to some new goal. As they 
are washed along the rocky bed of the stream they slowly rasp 
and wear it deeper. The valley will be deeper in the future ; 
it has been less deep in the past. 

In this little valley we see slow changes now in progress. We 
find also in the composition, the structure, and the attitude of the 
rocks, and the land forms to which they have been sculptured, 
the record of a long succession of past changes involving the 
origin of sand grains and their gathering and deposit upon the 
bottom of some ancient sea, the cementation of their layers into 
solid rock, the uplift of the rocks to form a land surface, and, 
last of all, the carving of a valley in the upland. 

Everywhere, in the fields, along the river, among the moun- 
tains, by the seashore, and in the desert, we may discover slow 
changes now in progress and the record of similar changes in the 
past. Everywhere we may catch gli mpses of a process of gradual 
change, which stretches backward into the past and forward 
into the future, by which the forms and structures of the face of 
the earth are continually built and continually destroyed. The 
science which deals with this long process is geology. Geology 
treats of the natural changes now taking place upon the earth 
and within it, the agencies which produce them, and the land 



4 THE ELEMENTS OF GEOLOGY 

forms and rock structures which result. It studies the changes 
of the present in order to be able to read the history of the 
earth's changes in the past. 

The various agencies which have fashioned the face of the 
earth may be divided into two general classes. In Part I we 
shall consider those which work upon the earth from without, 
such as the weather, running water, glaciers, the wind, and the 
sea. In Part II we shall treat of those agencies whose sources are 
within the earth, and among whose manifestations are volcanoes 
and earthquakes and the various movements of the earth's crust. 
As we study each agency we shall notice not only how it does 
its work, but also the records which it leaves in the rock struc- 
tures and the land forms which it produces. With this prepara- 
tion we shall be able in Part III to read in the records of the 
rocks the history of our planet and the successive forms of life 
which have dwelt upon it. 



PART I 

EXTERNAL GEOLOGICAL AGENCIES 

CHAPTER I* 
THE WORK OF THE WEATHER 

In our excursion to the valley with sandstone ledges we wit- 
nessed a process which is going forward in all lands. Every- 
where the rocks are crumbling away ; their fragments are creep- 
ing down hillsides to the stream ways and are carried by the 
streams to the sea, where they are rebuilt into rocky layers. 
When again the rocks are lifted to form land the process will 
begin anew; again they will crumble and creep down slopes 
and be washed by streams to the sea. Let us begin our study 
of this long cycle of change at the point where rocks disinte- 
grate and decay under the action of the weather. In studying 
now a few outcrops and quarries we shall learn a little of some 
common rocks and how they weather away. 

Stratification and jointing. At the sandstone ledges we saw 
that the rock was divided into parallel layers. The thicker 
layers are known as strata, and the thin leaves into which each 
stratum may sometimes be split are termed lamince. To a greater 
or less degree these layers differ from each other in fineness of 
grain, showing that the material has been sorted. The planes 
which divide them are called bedding planes. 

Besides the bedding planes there are other division planes, 
which cut across the strata from top to bottom. These are 

5 



6 



THE ELEMENTS OF GEOLOGY 



found in all rocks and are known as joints (Fig. 1). Two sets 
of joints, running at about right angles to each other, together 
with the bedding planes, divide the sandstone into quadrangular 
blocks. 

Sandstone. Examining a piece of sandstone we find it com- 
posed of grains quite like those of river sand or of sea beaches. 

Most of the grains 
are of a clear glassy 
mineral called 
quartz. These 
quartz grains are 
very hard and will 
scratch the steel of 
a knife blade. They 
are not affected by 
acid, and their 
broken surfaces are 
irregular like those 
of broken glass. 

The grains of 
sandstone are held 
together by some 
cement. This ma} 7 
be calcareous, con- 
sisting of soluble 
carbonate of lime. 
In brown sand- 
stones the cement is 
commonly ferrugi- 
nous^ hydrated iron oxide, or iron rust, forming the bond, 
somewhat as in the case of iron nails which have rusted together. 
The strongest and most lasting cement is siliceous, and sand 
rocks whose grains are closely cemented by silica, the chemical 
substance of which quartz is made, are known as quartzites. 




FIG. 1. Cliff of Sandstone, Ireland 

Note the horizontal bedding planes and the two sets 
of vertical joints which determine the cliff faces 



THE WORK OF THE WEATHER 



We are now prepared to understand how sandstone is affected 
by the action of the weather. On ledges where the rock is 
exposed to view its surface is more or less, discolored and the 
grains are loose and may be rubbed off with the finger. On 
gentle slopes the rock is covered with a soil composed of sand, 
which evidently is crumbled sandstone, and dark carbonaceous 
matter derived from the decay of vegetation. Clearly it is by 
the dissolving of the cement that the rock thus breaks down to 
loose sand. A piece of sand- 
stone with calcareous cement, 
or a bit of old mortar, which 
is really an artificial stone 
also made of sand cemented 
by lime, may be treated in a 
test tube with hydrochloric FIG. 2. Section of Limestone Quarry in 
acid to illustrate the process. Southeastern Wisconsin 

A limestone quarry. Here Scale, 1 in. 30 ft. a, red residual clay; 

inn, pitted surface of rotten limestone ; 




66, limestone divided into thin layers; 
c, thick layers of laminated limestone, 
the laminae being firmly cemented 
together; j, j, j, joints. Is 66 thin- 
layered because originally so laid, or 
because it has been broken up by 
weathering, although once like c thick- 
layered ? 



also we find the rock stratified 
and jointed (Fig. 2). On the 
quarry face the rock is dis- 
tinctly seen to be altered for 
some distance from its upper 
surface. Below the altered 
zone the rock is sound and is quarried for building; but the 
altered upper layers are too soft and broken to be used for this 
purpose. If the limestone is laminated, the laminse here have 
split apart, although below they hold fast together. Near the 
surface the stone has become rotten and crumbles at the touch, 
while on the top it has completely broken down to a thin layer 
of limestone meal, on which rests a fine reddish clay. 

Limestone is made of minute grains of carbonate of lime all 
firmly held together by a calcareous cement. A piece of the 
stone placed in a test tube with hydrochloric acid dissolves 
with brisk effervescence, leaving the insoluble impurities, which 



8 THE ELEMENTS OF GEOLOGY. 

were disseminated through it, at the bottom of the tube as a 
little clay. 

We can now understand the changes in the upper layers of 
the quarry. At the surface of the rock the limestone has com- 
pletely dissolved, leaving the insoluble residue as a layer of 
reddish clay. Immediately below the clay the rock has dis- 
integrated into meal where the cement between the limestone 
grains has been removed, while beneath this the laminae are 
split apart where the cement has been dissolved only along the 
planes of lamination where the stone is more porous. As these 
changes in the rock are greatest at the surface and diminish 
downward, we infer that they have been caused by agents 
working downward from the surface. 

At certain points these agencies have been more effective 
than elsewhere. The upper rock surface is pitted. Joints are 
widened as they approach the surface, and along these seams we 
may find that the rock is altered even down to the quarry floor. 

A shale pit. Let us now visit some pit where shale a 
laminated and somewhat hardened clay is quarried for the 
manufacture of brick. The laminae of this fine-grained rock may 
be as thin as cardboard in places, and close joints may break 
the rock into small rhombic blocks. On the upper surface we 
note that the shale has weathered to a clayey soil in which 
all traces of structure have been destroyed. The clay and the 
upper layers of the shale beneath it are reddish or yellow, while 
in many cases the color of the unaltered rock beneath is blue. 

The sedimentary rocks. The three kinds of layered rocks 
whose acquaintance we have made sandstone, limestone, and 
shale are the leading types of the great group of stratified, or 
sedimentary, rocks. This group includes all rocks made of sedi- 
ments, their materials having settled either in water upon the 
bottoms of rivers, lakes, or seas, or on dry land, as in the case 
of deposits made by the wind and by glaciers. Sedimentary 
rocks are divided into the f ragmental rocks which are made 



THE WORK OF THE WEATHER 



9 



of fragments, either coarse or fine and the far less common 
rocks which are constituted of chemical precipitates. 

The sedimentary rocks are divided according to their com- 
position into the following classes : 

1. The arenaceous, or quartz rocks, including beds of loose 
sand and gravel, sand- 
stone, quartzite, and 

conglomerate (a rock 
made of cemented 
rounded gravel or peb- 
bles). 

2. The calcareous, or 
lime rocks, including 
limestone and a soft 
white rock formed of 
calcareous powder 
known as chalk. 

3. The argillaceous, 
or clay rocks, including 
muds, clays, and shales. 

These three classes 
pass by mixture into 
one another. Thus FlG -' 3 - Conglomerate 

there are limy and clayey sandstones, sandy and clayey lime- 
stones, and sandy and limy shales. 

Granite. This familiar rock may be studied as an example 
of the second great group of rocks, the unstratified, or igne- 
ous rocks. These are not made of cemented sedimentary grains, 
but of interlocking crystals which have crystallized from a mol- 
ten mass. Examining a piece of granite, the most conspicuous 
crystals which meet the eye are those of feldspar. They are 
commonly pink, white, or yellow, and break along smooth cleav- 
age planes which reflect the light like tiny panes of glass. 
Mica may be recognized by its glittering plates, which split into 




10 THE ELEMENTS OF GEOLOGY 

thin elastic scales. A third mineral, harder than steel, break- 
ing along irregular surfaces like broken glass, we identify as 
quartz. 

How granite alters under the action of the weather may 
be seen in outcrops where it forms the bed rock, or country 
rock, underlying the loose formations of the surface, and in 
many parts of the northern states where granite bowlders and 
pebbles more or less decayed may be found in a surface sheet 
of stony clay called the drift. Of the different minerals com- 
posing granite, quartz alone remains unaltered. Mica weathers 
to detached flakes which have lost their elasticity. The feldspar 
crystals have lost their luster and hardness, and even have de- 
cayed to clay. Where long-weathered granite forms the coun- 
try rock, it often may be cut with spade or trowel for several 
feet from the surface, so rotten is the feldspar, and here the rock 
is seen to break down to a clayey soil containing grains of quartz 
and flakes of mica. 

These are a few simple illustrations of the surface changes 
which some of the common kinds of rocks undergo. The agen- 
cies by which these changes are brought about we will now 
take up under two divisions, chemical agencies producing 
rock decay and mechanical agencies producing rock disinte- 
gration. 

THE CHEMICAL WORK OF WATER 

As water falls on the earth in rain it has already absorbed 
from the air carbon dioxide (carbonic acid gas) and oxygen. As 
it sinks into the ground and becomes what is termed ground 
water, it takes into solution from the soil humus acids and 
carbon dioxide, both of which are constantly being generated 
there by the decay of organic matter. So both rain and ground 
water are charged with active chemical agents, by the help 
of which they corrode and rust and decompose all rocks to 
a greater or less degree. We notice now three of the chief 



THE WORK OF THE WEATHER 



11 



chemical processes concerned in weathering, solution, the 
fOTmaUon_-pf carbonates, and oxidation. 

Solution. Limestone, although so little affected by pure water 
that five thousand gallons would be needed to dissolve a sin- 
gle pound, is easily dissolved in water charged with carbon 
dioxide. In limestone regions well water is therefore "hard." 
On boiling the water for some time the carbon dioxide gas is 




FIG. 4. Surface of Limestone furrowed by Weathering, Montana 

expelled, the whole of the lime carbonate can no longer be held 
in solution, and much of it is thrown down to form a crust or 
"scale" in the kettle or in the tubes of the steam boiler. All 
waters which flow over limestone rocks or soak through them 
are constantly engaged in dissolving them away, and in the 
course of time destroy beds of vast extent and great thickness. 
The upper surface of limestone rocks becomes deeply pitted, 
as we saw in the limestone quarry, and where the mantle of 
waste has been removed it may be found so intricately furrowed 
that it is difficult to traverse (Fig. 4). 



12 THE ELEMENTS OF GEOLOGY 

Beds of rock salt buried among the strata are dissolved by 
seeping water, which issues in salt springs. Gypsum, a mineral 
composed of hydr.ated sulphate of lime, and so soft that it may 
be scratched with the finger nail, is readily taken up by water, 
giving to the water of wells and springs a peculiar hardness 
difficult to remove. 

The dissolving action of moisture may be noted on marble tomb- 
stones of some age, marble being a limestone altered by heat and pres- 
sure and composed of crystalline grains. By assuming that the date on 
each monument marks the year of its erection, one may estimate how 
many years on the average it has taken for weathering to loosen fine 
grains on the polished surface, so that they may be rubbed off with the 
finger, to destroy the polish, to round the sharp edges of tool marks in 
the lettering, and at last to open cracks and seams and break down the 
stone. We may notice also whether the gravestones weather more 
rapidly on the sunny or the shady side, and on the sides or on the top. 

The weathered surface of granular limestone containing shells shows 
them standing in relief. As the shells are made of crystalline carbonate 
of lime, we may infer whether the carbonate of lime is less soluble in 
its granular or in its crystalline condition. 

The formation of carbonates. In attacking minerals water 
does more than merely take them into solution. It decomposes 
them, forming new chemical compounds of which the carbonates 
are among the most important. Thus feldspar consists of the 
insoluble silicate of alumina, together witli certain alkaline 
silicates which are broken up by the action of water contain- 
ing carbon dioxide, forming alkaline carbonates. These carbon- 
ates are freely soluble and contribute potash and soda to soils 
and river waters. By the removal of the soluble ingredients of 
feldspar there is left the silicate of alumina, united with water 
or hydrated, in the condition of a fine plastic clay which, when 
white and pure, is known as kaolin and is used in the manu- 
facture of porcelain. Feldspathic rocks which contain no iron 
compounds thus weather to whitish crusts, and even apparently 
sound crystals of feldspar, when ground to thin slices and placed 



THE WORK OF THE WEATHER 13 

under the microscope, may be seen to be milky in color through- 
out because an internal change to kaolin has begun. 

Oxidation. Rocks containing compounds of iron weather to 
reddish crusts, and the seams of these rocks are often lined 
with rusty films. Oxygen and water have here united with the 
iron, forming hydrated iron oxide. The effects of oxidation may 
be seen in the alteration of 
many kinds of rocks and in 
red and yellow colors of soils 
and subsoils. 




Pyrite is a very hard mineral 
of a pale brass color, found 
in scattered crystals in many 
rocks, and is composed of iron 
and sulphur (iron sulphide). 
Under the attack of the weather 
it takes up oxygen, forming FlG ' 5 " Bowlder split by Heat and Cold, 

' , L . .. Western Texas 

iron sulphate (green vitriol), a 

soluble compound, and insoluble hydrated iron oxide, which as a 
mineral is known as limonite. Several large masses of iron sulphide 
were placed some years ago on the lawn in front of the National 
Museum at Washington. The mineral changed so rapidly to green 
vitriol that enough of this poisonous compound was washed into the 
ground to kill the roots of the surrounding grass. 

AGENTS OF MECHANICAL DISINTEGRATION 

Heat and cold. Rocks exposed to the direct rays of the sun 
become strongly heated by day and expand. After sunset they 
rapidly cool and contract. When the difference in temperature 
between day and night is considerable, the repeated strains of 
sudden expansion and contraction at last become greater than 
the rocks can bear, and they break, for the same reason that a 
glass cracks when plunged into boiling water (Fig. 5). 

Rocks are poor conductors of heat, and hence their surfaces 
may become painfully hot under the full blaze of the sun, while 



14 



THE ELEMENTS OF GEOLOGY 



the interior remains comparatively cool. By day the surface 
shell expands and tends to break loose from the mass of the 
stone. In cooling in the evening the surface shell suddenly con- 
tracts on the unyielding interior and in time is forced off in 
scales (Fig. 6). 

Many rocks, such as granite, are made up of grains of various 
minerals which differ in color and in their capacity to absorb 



FIG. 6. Bowlders scaling off under Heat and Cold, Western Texas 

heat, and which therefore contract and expand in different 
ratios. In heating and cooling these grains crowd against their 
neighbors and tear loose from them, so that finally the rock 
disintegrates into sand. 

The conditions for the destructive action of heat and cold 
are most fully met in arid regions when vegetation is wanting 
for lack of sufficient rain. The soil not being held together 
by the roots of plants is blown away over large areas, leav- 
ing the rocks bare to the blazing sun in a cloudless sky. The 



THE AVORK OF THE WEATHER 15 

air is dry, and the heat received by the earth by day is there- 
fore rapidly radiated at night into space. There is a sharp 
and sudden fall of temperature after sunset, and the rocks, 
strongly heated by day, are now chilled perhaps even to the 
freezing point. 

In the Sahara the thermometer has been known to fall 131 F. 
within a few hours. In the light air of the Pamir plateau in central 
Asia a rise of 90 F. has been recorded from seven o'clock in the 
morning to one o'clock in the afternoon. On the mountains of south- 
western Texas there are frequently heard crackling noises as the rocks 
of that arid region throw off scales from a fraction of an inch to four 
inches in thickness, and loud reports are made as huge bowlders split 
apart. Desert pebbles weakened by long exposure to heat and cold have 
been shivered to fine sharp-pointed fragments on being placed in sand 
heated to 180 F. Beds half a foot thick, forming the floor of lime- 
stone quarries in Wisconsin, have been known to buckle and arch and 
break to fragments under the heat of the summer sun. 

Frost. By this term is meant the freezing and thawing of 
water contained in the pores and crevices of rocks. All rocks 
are more or less porous and all contain more or less water 
in their pores. Workers in stone call this "quarry water," and 
speak of a stone as " green " before the quarry water has dried 
out. Water also seeps along joints and bedding planes and 
gathers in all seams and crevices. Water expands in freezing, ten 
cubic inches of water freezing to about eleven cubic inches of 
ice. As water freezes in the rifts and pores of rocks it expands 
with the irresistible force illustrated in the freezing and breaking 
of water pipes in winter. The first rift in the rock, perhaps too 
narrow to be seen, is widened little by little by the wedges of 
successive frosts, and finally the rock is broken into detached 
blocks, and these into angular chip-stone by the same process. 

It is on mountain tops and in high latitudes that the effects of frost 
are most plainly seen. " Every summit," says Whymper, " amongst the 
rock summits upon which I have stood has been nothing but a piled-up 



16 



THE ELEMENTS OF GEOLOGY 



heap of fragments " (Fig. 7). In Iceland, in Spitzbergen, in Kamchatka, 
and in other frigid lands large areas are thickly strewn with sharp-edged 
fragments into which the rock has been shattered by frost. 

Organic agents. We must reckon the roots of plants and 
trees among the agents which break rocks into pieces. The 
tiny rootlet in its search for food and moisture inserts itself 

into some minute 
rift, and as it grows 
slowly wedges the 
rock apart. 

Moreover, the 
acids of the root cor- 
rode the rocks with 
which they are in 
contact. One may 
sometimes find in 
the soil a block of 
limestone wrapped 




FIG. 7. Rocks broken by Frost, Summit of the 
Eggischhorn, Switzerland 



in a mesh of roots, 
each of which lies in a little furrow where it has eaten into 
the stone. 

Bootless plants called lichens often cover and corrode rocks 
as yet bare of soil; but where lichens are destroying the rock 
less rapidly than does the weather, they serve in a way as a 
protection. 

Conditions favoring disintegration and decay. The disinte- 
gration of rocks under frost and temperature changes goes on 
most rapidly in cold and arid climates, and where vegetation 
is scant or absent. On the contrary, the decay of rocks under 
the chemical action of water is favored by a warm, moist 
climate and abundant vegetation. Frost and heat and cold can 
only act within the few feet from the surface to which the 
necessary temperature changes are limited, while water pene- 
trates and alters the rocks to great depths. 



THE WORK OF THE WEATHER 17 

The pupil may explain 

In what ways the presence of joints and bedding planes assists in the 
breaking up and decay of rocks under the action of the weather. 

Why it is a good rule of stone masons never to lay stones on edge, 
but always on their natural bedding planes. 

Why stones fresh from the quarry sometimes go to pieces in early 
winter, when stones which have been quarried for some months remain 
uninjured. 

Why quarrymen in the northern states often keep their quarry floors 
flooded during winter. 

Why laminated limestone should not be used for curbstone. 

Why rocks composed of layers differing in fineness of grain and in 
ratios of expansion do not make good building stone. 

Fine-grained rocks with pores so small that capillary attraction keeps 
the water which they contain from readily draining away are more apt to 
hold their pores ten elevenths full of water than are rocks whose pores 
are larger. W r hich, therefore, are more likely to be injured by frost? 

Which is subject to greater temperature changes, a dark rock or one 
of a light color ? the north side or the south side of a valley ? 

THE MANTLE OF ROCK WASTE 

We .have seen that rocks are everywhere slowly wasting 
away. They are broken in pieces by frost, by tree roots, and by 
heat and cold. They dissolve and decompose under the chemical 
action of water and the various corrosive substances which it 
contains, leaving their insoluble residues as residual clays and 
sands upon the surface. As a result there is everywhere form- 
ing a mantle of rock waste which covers the land. It is well to 
imagine how the country would appear were this mantle with 
its soil and vegetation all scraped away or had it never been 
formed. The surface of the land would then be everywhere of 
bare rock as unbroken as a quarry floor. 

The thickness of the mantle. In any locality the thickness 
of the mantle of rock waste depends as much on the rate at 
which it is constantly being removed as on the rate at which 



18 THE ELEMENTS OF GEOLOGY 

it is forming. On the face of cliffs it is absent, for here waste 
is removed as fast as it is made. Where waste is carried away 
more slowly than it is produced, it accumulates in time to great 
depth. 

The granite of Pikes Peak is disintegrated to a depth of twenty feet. 
In the city of Washington granite rock is so softened to a depth of eighty 
feet that it can be removed with pick and shovel. About Atlanta, 
Georgia, the rocks are completely rotted for one hundred feet from the 
surface, while the beginnings of decay may be noticed at thrice that 
depth. In places in southern Brazil the rock is decomposed to a depth 
of four hundred feet. 

In southwestern Wisconsin a reddish residual clay has an average 
depth of thirteen feet on broad uplands, where it has been removed to 
the least extent. The country rock on which it rests is a limestone 
with about ten per cent of insoluble impurities. At least how thick, 
then, was that portion of the limestone which has rotted down to 
the clay? 

Distinguishing characteristics of residual waste. We must 
learn to distinguish waste formed in place by the action of the 
weather from the products of other geological agencies. Resid- 
ual waste is unstratified. It contains no substances which 
have not been derived from the weathering of the parent rock. 
There is a gradual transition from residual waste into the 
un weathered rock beneath. Waste resting on sound rock evi- 
dently has been shifted and was not formed in place. 

In certain regions of southern Missouri the land is covered with a 
layer of broken flints and red clay, while the country rock is limestone. 
The limestone contains nodules of flint, and we may infer that it has 
been by the decay and removal of thick masses of limestone that the 
residual layer of clay and flints has been left upon the surface. Flint is 
a form of quartz, dull-lustered, usually gray or blackish in color, and 
opaque except on thinnest edges, where it is translucent. 

Over much of the northern states there is spread an unstratified stony 
clay called the drift. It often rests on sound rocks. It contains grains 
of sand, pebbles, and bowlders composed of many different minerals and 



THE WORK OF THE WEATHER 19 

rocks that the country rock cannot furnish. Hence the drift cannot 
have been formed by the decay of the rock of the region. A shale or 
limestone, for example, cannot waste to a clay containing granite peb- 
bles. The origin of the drift will be explained in subsequent chapters. 

The differences in rocks are due more to their soluble than to 
their insoluble constituents. The latter are few in number and 
are much the same in rocks of widely different nature, being 
chiefly quartz, silicate of alumina, and iron oxide. By the 
removal of their soluble parts very many and widely different 
rocks rot down to a residual clay gritty with particles of quartz 
and colored red or yellow with iron oxide. 

In a broad way the changes which rocks undergo in weather- 
ing are an adaptation to the environment in which they find 
themselves at the earth's surface, an environment different 
from that in which they were formed under sea or under ground. 
In open air, where they are attacked by various destructive 
agents, few of the rock-making minerals are stable compounds 
except quartz, the iron oxides, and the silicate of alumina ; and 
so it is to one or more of these comparatively insoluble sub- 
stances that most rocks are reduced by long decay. 

Which produces a mantle of finer waste, frost or chemical decay? 
which a thicker mantle? In what respects would you expect that the 
mantle of waste would differ in warm humid lands like India, in frozen 
countries like Alaska, and in deserts such as the Sahara? 

The soil. The same agencies which produce the mantle of 
waste are continually at work upon it, breaking it up into finer 
and finer particles and causing its more complete decay. Thus 
on the surface, where the waste has weathered longest, it is 
gradually made fine enough to support the growth of plants, 
and is then known as soil. The coarser waste beneath is some- 
times spoken of as subsoil. Soil usually contains more or less 
dark, carbonaceous, decaying organic matter, called humus, and 
is then often termed the humus layer. Soil forms not only 
on waste produced in place from the rock beneath, but also on 



20 THE ELEMENTS OF GEOLOGY 

materials which have been transported, such as sheets of glacial 
drift and river deposits. 

Until rocks are reduced to residual clays the work of the weather is 
more rapid and effective on the fragments of the mantle of waste than 
on the rocks from which waste is being formed. Why ? 

Any fresh excavation of cellar or cistern, or cut for road or 
railway, will show the characteristics of the humus layer. It 
may form only a gray film 011 the surface, or we may find it a 
layer a foot or more thick, dark, or even black, above, and grow- 
ing gradually lighter in color as it passes by insensible gradations 
into the subsoil. In some way the decaying vegetable matter 
continually forming on the surface has become mingled with 
the material beneath it. 

How humus and the subsoil are mingled. The mingling of 
humus and the subsoil is brought about by several means. The 
roots of plants penetrate the waste, and when they die leave their 
decaying substance to fertilize it. Leaves and stems falling on 
the surface are turned under by several agents. Earthworms and 
other animals whose home is in the waste drag them into their 
burrows either for food or to line their nests. Trees overthrown 
by the wind, roots and all, turn over the soil and subsoil and 
mingle them together. Bacteria also work in the waste and 
contribute to its enrichment. The animals living in the mantle 
do much in other ways toward the making of soil. They bring 
the coarser fragments from beneath to the surface, where the 
waste weathers more rapidly. Their burrows allow air and 
water to penetrate the waste more freely and to affect it to 
greater depths. 

Ants. In the tropics the mantle of waste is worked over chiefly by 
ants. They excavate underground galleries and chambers, extending 
sometimes as much as fourteen feet below the surface, and build mounds 
which may reach as high above it. In some parts of Paraguay and 
southern Brazil these mounds, like gigantic potato hills, cover tracts of 
considerable area. 



THE WORK OF THE WEATHER 21 

In search for its food the dead wood of trees the so-called white 
ant constructs runways of earth about the size of gas pipes, reaching 
from the base of the tree to the topmost branches. On the plateaus of 
central Africa explorers have walked for miles through forests every 
tree of which was plastered with these galleries of mud. Each grain of 
earth used in their construction is moistened and cemented by slime 
as it is laid in place by the ant, and is thus acted on by organic chem- 
ical agents. Sooner or later these galleries are beaten down by heavy 
rains, and their fertilizing substances are scattered widely by the winds. 

Earthworms. In temperate regions the waste is worked over largely 
by earthworms. In making their burrows worms swallow earth in 
order to extract from it any nutritive organic matter which it may 
contain. They treat it with their digestive acids, grind it in their stony 
gizzards, and void it in castings on the surface of the ground. It was 
estimated by Darwin that in many parts of England each year, on every 
acre, more than ten tons of earth pass through the bodies of earthworms 
and are brought to the surface, and that every few years the entire soil 
layer is thus worked over by them. 

In all these ways the waste is niade fine and stirred and 
enriched. Grain by grain the subsoil with its fresh mineral 
ingredients is brought to the surface, and the rich organic 
matter which plants and animals have taken from the atmos- 
phere is plowed under. Thus Nature plows and harrows on 
"the great world's farm" to make ready and ever to renew a 
soil fit for the endless succession of her crops. 

The world processes by which rocks are continually wasting 
away are thus indispensable to the life of plants and animals. 
The organic world is built on the ruins of the inorganic, and 
because the solid rocks have been broken down into soil men 
are able to live upon the earth. 

Solar energy. The source of the energy which accomplishes all 
this necessary work is the sun. It is the radiant energy of the 
sun which causes the disintegration of rocks, which lifts vapor 
into the atmosphere to fall as rain, which gives life to plants 
and animals. Considering the earth in a broad way, we may 
view it as a globe of solid rock, the lithosphere, surrounded 



22 THE ELEMENTS OF GEOLOGY 

by two mobile envelopes : the envelope of air, the atmosphere , 
and the envelope of water, the hydrosphere. Under the 
action of solar energy these envelopes are in constant motion. 
Water from the hydrosphere is continually rising in vapor into 
the atmosphere, the air of the atmosphere penetrates the hydro- 
sphere, for its gases are dissolved in all waters, and both 
air and water enter and work upon the solid earth. By their 
action upon the lithosphere they have produced a third envelope, 
the mantle of rock waste. 

This envelope also is in movement, not indeed as a whole, 
but particle by particle. The causes which set its particles in 
motion, and the different forms which the mantle comes to 
assume, we will now proceed to study. 

MOVEMENTS OF THE MANTLE OF ROCK WASTE 

At the sandstone ledges which we first visited we saw not 
only that the rocks were crumbling away, but also that grains 
and fragments of them were creeping down the slopes of the 
valley to the stream and were carried by it onward toward the sea. 
This process is going on everywhere. Slowly it may be, and 
with many interruptions, but surely, the waste of the land moves 
downward to the sea. We may divide its course into two 
parts, the path to the stream, which we will now consider, 
and its carriage onward by the stream, which we will defer to 
a later chapter. 

Gravity. The chief agent concerned in the movement of 
waste is gravity. Each particle of waste feels the unceasing 
downward pull of the earth's mass and follows it when free to 
do so. All agencies which produce waste tend to set its particles 
free and in motion, and therefore cooperate with gravity. On 
cliffs, rocks fall when wedged off by frost or by roots of trees, 
and when detached by any other agency. On slopes of waste, 
water freezes in chinks between stones, and in pores between 



THE WORK OF THE WEATHER 23 

particles of soil, and wedges them apart. Animals and plants 
stir the waste, heat expands it, cold contracts it, the strokes of 
the raindrops drive loose particles down the slope and the wind 
lifts and lets them fall. Of all these movements, gravity assists 
those which are downhill and retards those which are uphill. 
On the whole, therefore, the downhill movements prevail, and 
the mantle of waste, block by block and grain by grain, creeps 
along the downhill path. 

A slab of sandstone laid on another of the same kind at an angle of 
17 and left in the open air was found to creep down the slope at 
the rate of a little more than a millimeter a month. Explain why it 
did so. 

Rain. The most efficient agent in the carriage of waste to 
the streams is the rain. It moves particles of soil by the force 
of the blows of the falling drops, and washes them down all 
slopes to within reach of permanent streams. On surfaces unpro- 
tected by vegetation, as on plowed fields and in arid regions, 
the rain wears furrows and gullies both in the mantle of waste 
and in exposures of unaltered rock (Fig. 17). 

At the foot of a hill we may find that the soil has accumulated by creep 
and wash to the depth of several feet ; while where the hillside is steepest 
the soil may be exceedingly thin, or quite absent, because removed 
about as fast as formed. Against the walls of an abbey built on a slope 
in Wales seven hundred years ago, the creeping waste has gathered on the 
uphill side to a depth of seven feet. The slow-flowing sheet of waste is 
often dammed by fences and walls, whose uphill side gathers waste in a 
few years so as to show a distinctly higher surface than the downhill 
side, especially in plowed fields .where the movement is least checked 
by vegetation. 

Talus. At tjie foot of cliffs there is usually to be found a 
slope of rock fragments which clearly have fallen from above 
(Fig. 8). Such a heap of waste is known as talus. The amount 
of talus in any place depends both on the rate of its formation 
and the rate of its removal. Talus forms rapidly in climates 




24 



THE WORK OF THE WEATHER 25 

where mechanical disintegration is most effective, where rocks 
are readily broken into blocks because closely jointed and 
thinly bedded rather than massive, and where they are firm 
enough to be detached in fragments of some size instead of in 
fine grains. Talus is removed slowly where it decays slowly, 
either because of the climate or the resistance of the rock. It 
may be rapidly removed by a stream flowing along its base. 

In a moist climate a soluble rock, such as massive limestone, 
may form talus little if any faster than the talus weathers away. 
A loose-textured sandstone breaks down into incoherent sand 
grains, which in dry climates, where unprotected by vegetation, 
may be blown away as fast as. they fall, leaving the cliff bare to 
fche base. Cliffs of such slow-decaying rocks as quartzite and 
granite when closely jointed accumulate talus in large amounts. 

Talus slopes may be so steep as to reach the angle of repose, 
Le. the steepest angle at which the material will lie. This 
angle varies with different materials, being greater with coarse 
and angular fragments than with fine rounded grains. Sooner 
or later a talus reaches that equilibrium where the amount 
removed from its surface just equals that supplied from the 
cliff above. As the talus is removed and 
weathers away its slope retreats together 
with the retreat of the cliff, as seen in 
Figure 9. 

Graded slopes. Where rocks weather FIG. 9. Diagram illustrat- 

faster than their waste is carried away, ^ Retreat of Cliff, c, 

/' and Talus, t 
the waste comes at last to cover all rocky 

ledges. On the steeper slopes it is coarser and in more rapid 
movement than on slopes more gentle, but mountain sides and 
hills and plains alike come to be mantled with sheets of waste 
which everywhere is creeping toward the streams. Such un- 
broken slopes, worn or built to the least inclination at which 
the waste supplied by weathering can be urged onward, are 
known as graded slopes. 




26 



THE ELEMENTS OF GEOLOGY 



Of far less importance than the silent, gradual creep of waste, 
which is going on at all times everywhere about us, are the 
startling local and spasmodic movements which we are now to 
describe. 

Avalanches. On steep mountain sides the accumulated snows 
of winter often slip and slide in avalanches to the valleys below. 




FIG. 10. A Landslide, Quebec 

These rushing torrents of snow sweep their tracks clean of 
waste and are one of Nature's normal methods of moving it 
along the downhill path. 

Landslides. Another common and abrupt method of deliver- 
ing waste to streams is by slips of the waste mantle in large 
masses. After long rains and after winter frosts the cohesion 
between the waste and the sound rock beneath is loosened bv 




THE WORK OF THE WEATHER 27 

seeping water underground. The waste slips on the rock sur- 
face thus lubricated and plunges down the mountain side in a 
swift roaring torrent of mud and stones. 

We may conveniently mention here a second type of land- 
slide, where masses of solid rock as well as the mantle of waste 
are involved in the sudden movement. 
Such slips occur when valleys have 
been rapidly deepened by streams or 
glaciers and their sides have not yet 

been graded. A favorable condition _ 

FIG. 11. Diagram illustrating 

is where the strata dip (i.e. incline Conditions favorable to a 
downwards) towards the valley (Fig. Landslide 
11), or are broken by joint planes im, limestone dipping toward 
Clipping in the Same direction. The Bailey of river, r ; sh, shale 

upper layers, including perhaps the entire mountain side, have 
been cut across by the valley trench and are left supported only 
on the inclined surface of the underlying rocks. Water may 
percolate underground along this surface and loosen the cohesion 
between the upper and the underlying strata by converting the 
upper surface of a shale to soft wet clay, by dissolving layers 
of a limestone, or by removing the cement of a sandstone and 
converting it into loose sand. When the inclined surface is 
thus lubricated the overlying masses may be launched into the 
valley below. The solid rocks are broken and crushed in slid- 
ing and converted into waste consisting, like that of talus, 
of angular unsorted fragments, blocks of all sizes being min- 
gled pellmell with rock meal and dust. The principal effects of 
landslides may be gathered from the following examples. 

At Gohna, India, in 1893, the face of a spur four thousand feet high, 
of the lower ranges of the Himalayas, slipped into the gorge of the 
headwaters of the Ganges River in successive rock falls which lasted for 
three days. Blocks of stone were projected for a mile, and clouds of 
limestone dust were spread over the surrounding country. The debris 
formed a dam one thousand feet high, extending for two miles along the 



28 



THE ELEMENTS OF GEOLOGY 



valley. A lake gathered behind this barrier, gradually rising until it 
overtopped it in a little less than a year. The upper portion of the 
dam then broke, and a terrific rush of water swept down the valley in a 
wave which, twenty miles away, rose one hundred and sixty feet in height. 
A narrow lake is still held by the strong base of the dam. 

In 1896, after forty days of incessant rain, a cliff of sandstone slipped 
into the Yangtse River in China, reducing the width of the channel to 
eighty yards and causing formidable rapids. 

At Flims, in Switzerland, a prehistoric landslip flung a dam eighteen 
hundred feet high across the headwaters of the Rhine. If spread 
evenly over a surface of twenty-eight square miles, the material would 

cover it to a depth of six 
hundred and sixty feet. 
The barrier is not yet en- 
tirely cut away, and several 
lakes are held in shallow 
basins on its hummocky 
surface. 

A slide from the pre- 
cipitous river front of the 
citadel hill of Quebec, in 
1889, dashed across Cham- 
plain Street, wrecking a 
number of houses and caus- 
ing the death of forty-five 
persons. The strata here 
are composed of steeply 
dipping slate (Fig. 10). 
In lofty mountain ranges there may not be a single valley without 
its traces of landslides, so common there is this method of the movement 
of waste, and of building to grade over-steepened slopes. 




FIG. 12. Bowlders of Weathering, Granite 
Quarry, Cape Ann, Massachusetts 

The rock is divided into blocks by horizontal 
and vertical joint planes. How do the bowl- 
ders of the upper ledge differ in shape from 
those beneath, and why ? 



KOCK SCULPTURE BY WEATHERING 

We are now to consider a few of the forms into which rock 
masses are carved by the weather. 

Bowlders of weathering. In many quarries and outcrops we 
may see that the blocks into which one or more of the uppermost 



THE WORK OF THE WEATHER 



29 



layers have been broken along their 
joints and bedding planes are no 
longer angular, as are those of the 
layers below. The edges and corners 
of these blocks have been worn away 
by the weather. Such rounded cores, 
known as bowlders of weathering, 
are often left to strew the surface. 

Differential weathering. This 
term covers all cases in which a rock FIG m Differential Weather . 
mass weathers differently in differ- ing on a Monument, Colo- 
ent portions. Any weaker spots or T ^ 
layers are etched out on the surface, leaving the more resistant 
in relief. Thus massive limestones become pitted where the 





FIG. 14. Honeycombed Limestone, Iowa 

weather drills out the weaker portions. In these pits, when 
once they are formed, moisture gathers, a little soil collects, 
vegetation takes root, and thus they are further enlarged until 
the limestone may be deeply honeycombed. 




30 



THE WORK OF THE WEATHER 31 

On the sides of canyons, and elsewhere where the edges of 
strata are exposed, the harder layers project as cliffs, while 
the softer weather back to slopes covered with the talus of the 
harder layers above them. It is convenient to call the former 
cliff makers and the latter slope makers (Fig. 15). 

Differential weathering plays a large part hi the sculpture 
of the land. Areas of weak rock are wasted to plains, while 
areas of hard rock adjacent are still left as hills and mountain 
ridges, as in the valleys and mountains of eastern Pennsylvania. 
But in such instances the lowering of the surface of the weaker 




.FIG. 16. A Small Mesa, New Mexico 

rock is also due to the wear of streams, and especially to the 
removal by them from the land of the waste which covers and 
protects the rocks beneath. 

Rocks owe their weakness to several different causes. Some, such as 
beds of loose sand, are soft and easily worn by rains ; some, as lime- 
stone and gypsum for example, are soluble. Even hard insoluble rocks 
are weak under the attack of the weather when they are closely divided 
by joints and bedding planes and are thus readily broken up into blocks 
by mechanical agencies. 

Outliers and monuments. As cliffs retreat under the attack 
of the weather, portions are left behind where the rock is 
more resistant or where the attack for any reason is less severe. 
Such remnant masses, if large, are known as outliers. When 



THE WORK OF THE WEATHER 



33 



flat-topped, because of the protection of a resistant horizontal 
capping layer, they are termed mesas (Fig. 16), a term applied 
also to the flat-topped portions of dissected plateaus (Fig. 129). 
Retreating cliffs may fall back a number of miles behind their 
outliers before the latter are finally consumed. 

Monuments are smaller masses and may be but partially 
detached from the cliff face. In the breaking down of sheets 
of horizontal strata, outliers grow smaller and smaller and are 
reduced to massive rectangular monuments resembling castles 
(Fig. 17). The rock castle falls 
into ruin, leaving here and there 
an isolated tower; the tower 
crumbles to a lonely pillar, soon 
to be overthrown. The various 
and often picturesque shapes of 
monuments depend on the kind 
of rock, the attitude of the 
strata, and the agent by which 
they are chiefly carved. Thus 
pillars may have a capital formed 
of a resistant stratum. Monu- 
ments may be undercut and 
come to rest on narrow pedes- 
tals, wherever they weather more rapidly near the ground, either 
because of the greater moisture there, or in arid climates 
because worn at their base by drifting sands. 

Stony clays disintegrating under the rain often contain bowlders 
which protect the softer material beneath from the vertical blows of 
raindrops, and thus come to stand on pedestals of some height 
(Fig. 19). One may sometimes see on the ground beneath dripping eaves 
pebbles left in the same way, protecting tiny pedestals of sand. 

Mountain peaks and ridges. Most mountains have been carved 
out of great broadly uplifted folds and blocks of the earth's 
crust. Running water and glacier ice have cut these folds and 




FIG. 18. Undercut Monuments, 
Colorado 



34 



THE ELEMENTS OF GEOLOGY 



blocks into masses divided by deep valleys ; but it is by the 
weather, for the most part, that the masses thus separated have 
been sculptured to the present forms of the individual peaks 

and ridges. 

Frost and heat and cold sculpture 
high mountains to sharp, tusklike 
peaks and ragged, serrate crests, 
where their waste is readily removed 
(Fig. 8). 

The Matterhorn of the Alps is a fa- 
mous example of a mountain peak whose 
carving by the frost and other agents is 
in active progress. On its face " scarcely 
a rock anywhere is firmly attached," and 
the fall of loosened stones is incessant. 
Mountain climbers who have camped at 
its base tell how huge rocks from time 
to time come leaping down its precipices, 
followed by trains of dislodged smaller 
fragments and rock dust ; and how at 
night one may trace the course of the 
bowlders by the sparks which they strike 
from the mountain walls. Mount Assini- 
boine, Canada (Fig. 20), resembles the 
Matterhorn in form and has been carved 
by the same agencies. 

" The Needles " of Arizona are ex- 




FIG. 19. 



Roosevelt Column. 
Idaho 



An erosion pillar 70 feet high. 
How was it produced? Why 
quadrangular? What does it amples of sharp mountain peaks in a 
show as to the recent height of wa rm arid region sculptured chiefly by 
the hillside surf ace? 

temperature changes. 

Chemical decay, especially when carried on beneath a cover of waste 
arid vegetation, favors the production of rounded knobs and dome-shaped 
mountains. 

The weather curve. We have seen that weathering reduces 
the angular block quarried by the frost to a rounded bowlder by 
chipping off its corners and smoothing away its edges. In much 



THE WORK OF THE WEATHER 



35 



the same way weathering at last reduces to rounded hills the 
earth blocks cut by streams or formed in any other way. High 




Mount Assiniboine, Canada 



mountains may at first be sculptured by the weather to savage 
peaks (Fig. 181), but toward the end of their life history they 




FIG. 21. Big Round Top and Little Round Top, 
Gettysburg, Pennsylvania 

wear down to rounded hills (Fig. 182). The weather curve, 
which may be seen on the summits of low hills (Fig. 21), is 
convex upward. 



36 THE ELEMENTS OF GEOLOGY 

In Figure 22, representing a cubic block of stone whose faces are a 
yard square, how many square feet of surface are exposed to the 
weather by a cubic foot at a corner a ; by one situated in the middle 
of an edge & ; by one in the center of a side c ? How much faster 
will a and b weather than c, and what will be the effect on the shape of 
the block ? 

The cooperation of various agencies in rock sculpture. For 

the sake of clearness it is necessary to describe the work of each 
geological agent separately. We must not forget, however, that 
in Nature no agent works independently and alone; that every 
result is the outcome of a long chain of causes. Thus, in order 
that the mountain peak may be carved by 
the agents of disintegration, the waste must 
be rapidly removed, a work done by many 
agents, including some which we are yet to 
study ; and in order that the waste may be 
removed as fast as formed, the region must 
first have been raised well above the level of 
the sea, so that the agents of transportation could do their work 
effectively. The sculpture of the rocks is accomplished only by 
the cooperation of many forces. 

The constant removal of waste from the surface by creep 
and wash and carriage by streams is of the highest impor- 
tance, because it allows the destruction of the land by means of 
weathering to go on as long as any land remains above sea 
level. If waste were not removed, it would grow to be so thick 
as to protect the rock beneath from further weathering, and 
the processes of destruction which we have studied would be 
brought to an end. The very presence of the mantle of waste 
over the land proves that on the whole rocks weather more 
rapidly than their waste is removed. The destruction of the 
land is going on as fast as the waste can be carried away. 

We have now learned to see in the mantle of waste the 
record of the destructive action of the agencies of weathering 



THE WORK OF THE WEATHER 



37 



on the rocks of the land surface. Similar records we shall find 
buried deeply among the rocks of the crust in old soils and in 
rocks pitted and decayed, telling of old land surfaces long 
wasted by the weather. Ever since the dry land appeared these 
agencies have been as now quietly and unceasingly at work 
upon it, and have ever been the chief means of the destruction 




FIG. 23. Mount Sneffels, Colorado 

Describe and account for what you see in this view. What changes may 
the mountain be expected to undergo in the future from the agencies 
now at work upon it ? 

of its rocks. The vast bulk of the stratified rocks of the earth's 
crust is made up almost wholly of the waste thus worn from 
ancient lands. 

In studying the various geological agencies we must remem- 
ber the almost inconceivable times in which they work. The 
slowest process when multiplied by the immense time in which 
it is carried on produces great results. The geologist looks upon 
the land forms of the earth's surface as monuments which 
record the slow action of weathering and other agents during 
the ages of the past. The mountain peak, the rounded hill, the 



38 THE ELEMENTS OF GEOLOGY 

wide plain which lies where hills and mountains once stood, 
tell clearly of the great results which slow processes will reach 
when given long time in which to do their work. We should 
accustom ourselves also to think of the results which weather- 
ing will sooner or later bring to pass. The tombstone and the 
bowlder of the field, which each year lose from their surfaces 
a few crystalline grains, must in time be wholly destroyed. 
The hill whose rocks are slowly rotting underneath a cover of 
waste must become lower and lower as the centuries and mil- 
lenniums come and go, and will finally disappear. Even the 
mountains are crumbling away continually, and therefore are 
but fleeting features of the landscape. 



CHAPTER II 
THE WORK OF GROUND WATER 

Land waters. We have seen how large is the part that water 
plays at and near the surface of the land in the processes of 
weathering and in the slow movement of waste down all 
slopes to the stream ways. We now take up the work of water 
as it descends beneath the ground, a corrosive agent still, 
and carrying in solution as its load the invisible waste of rocks 
derived from their soluble parts. 

Land waters have their immediate source in the rainfall. 
By the heat of the sun water is evaporated from the reservoir 
of the ocean and from moist surfaces everywhere. Mingled as 
vapor with the air, it is carried by the winds over sea and land, 
and condensed it returns to the earth as rain or snow. That 
part of the rainfall which descends on the ocean does not con- 
cern us, but that which falls on the land accomplishes, as it 
returns to the sea, the most important work of all surface 
geological agencies. 

The rainfall may be divided into three parts : the first dries 
up, being discharged into the air by evaporation either directly 
from the soil or through vegetation ; the second runs off over 
the surface to flood the streams ; the third soaks in the ground 
and is henceforth known as ground or underground water. 

The descent of ground water. Seeping through the mantle 
of waste, ground water soaks into the pores and crevices of 
the underlying rock. All rocks of the upper crust of the 
are more or less porous, and all drink in water. Impervioud 
rocks, such as granite, clay, and shale, have pores so minute 
that the water which they take in is held fast within them by 



40 THE ELEMENTS OF GEOLOGY 

capillary attraction, and none drains through. Pervious rocks, 
on the other hand, such as many sandstones, have pore spaces 
so large that water niters through them more or less freely. 
Besides its seepage through the pores of pervious rocks, water 
passes to lower levels through the joints and cracks by which 
all rocks near the surface are broken. 

Even the closest-grained granite has a pore space of 1 in 400, while 
sandstone may have a pore space of 1 in 4. Sand is so porous that it 
may absorb a third of its volume of water, and a loose loam even as 
much as one half. 

The ground- water surface is the name given the upper surface 
of ground water, the level below which all rocks are saturated. 
In dry seasons the ground-water surface sinks. For ground 

water is constantly 
seeping downward 
under gravity, it is 
evaporated in the 
FIG. 24. Diagram illustrating the Relation of the waste, and its mois- 
Ground-Water Surface to the Surface of the ture ^ ^^ 
Ground 

ward by capillarity 

The dotted line represents the ground-water surface, J J 

and arrows indicate the direction of the movements and the roots of 
of ground water, m, marsh ; w, well ; r, river pknts t() the gurf ace 

to be evaporated in the air. In wet seasons these constant 
losses are more than made good by fresh supplies from that 
part of the rainfall which soaks into the ground, and the ground- 
water surface rises. 

In moist climates the ground-water surface (Fig. 24) lies, as 
a rule, within a few feet of the land surface and conforms to 
it iii a general way, although with slopes of less inclination 
than those of the hills and valleys. In dry climates permanent 
ground water may be found only at depths of hundreds of feet. 
Ground water is held at its height by the fact that its circula- 
tion is constantly impeded by capillarity and friction. If it 
were as free to drain away as are surface streams, it would 




THE WORK OF GROUND WATER 



41 



sink soon after a rain to the level of the deepest valleys of 
the region. 

Wells and springs. Excavations made in permeable rocks 
below the ground-water surface fill to its level and are known 
as wells. Where valleys cut this surface permanent streams are 
formed, the water either oozing forth along ill-defined areas or 
issuing at definite points called springs, where it is concentrated 
by the structure of the 
rocks. A level tract 
where the ground- water 
surface coincides with 
the surface of the 
ground is a swamp or 
marsh. 

By studying a spring 
one may learn much of 
the ways and work of 
ground water. Spring 
water differs from that 
of the stream into 
w^hich it flows in several 
respects. If we test the 
spring with a ther- 
mometer during succes- 
sive months, we shall find that its temperature remains much 
the same the year round. In summer it is markedly cooler than 
the stream ; in winter it is warmer and remains unfrozen while 
the latter perhaps is locked in ice. This means that its under- 
ground path must lie at such a distance from the surface that 
it is little affected by summer's heat and winter's cold. 

While the stream is often turbid with surface waste washed 
into it by rains, the spring remains clear ; its water has been 
filtered during its slow movement through many small under- 
ground passages and the pores of rocks. Commonly the spring 




FIG. 25. A Spring, Kansas 

Is the rock over which the spring discharges 
pervious or impervious? 



42 THE ELEMENTS OF GEOLOGY 

differs from the stream in that it carries a far larger load of 
dissolved rock. Chemical analysis proves that streams contain 
various minerals in solution, but these are usually in quantities 
so small that they are not perceptible to the taste or feel. But 
the water of springs is often well charged with soluble minerals ; 
in its slow, long journey underground it has searched out the sol- 
uble parts of the rocks through which it seeps and has dissolved 
as much of them as it could. When -spring water is boiled away, 
the invisible load which it has carried is left behind, and in 
composition is found to be practically identical with that of 
the soluble ingredients of the country rock. Although to some 
extent the soluble waste of rocks is washed down surface slopes 
by the rain, by far the larger part is carried downward by 
ground water and is delivered to streams by springs. 

In limestone regions springs are charged with calcium carbonate (the 
carbonate of lime), and where the limestone is magnesian they contain 
magnesium carbonate also. Such waters are " hard "; when used in wash- 
ing, the minerals which they contain combine with the fatty acids of 
soap to form insoluble curdy compounds. When springs rise from rocks 
containing gypsum they are hard with calcium sulphate. In granite 
regions they contain more or less soda and potash from the decay of 
feldspar. 

The flow of springs varies much less during the different 
seasons of the year than does that of surface streams. So slow 
is the movement of ground water through the rocks that even 
during long droughts large amounts remain stored above the 
levels of surface drainage. 

Movements of ground water. Ground water is in constant 
movement toward its outlets. Its rate varies according to many 
conditions, but always is extremely slow. Even through loose 
sands beneath the beds of rivers it sometimes does not exceed 
a fifth of a mile a year. 

In any region two zones of flow may be distinguished. The 
upper zone of flow extends from the ground-water surface 



THE WORK OF GROUND WATER 



43 



downward through the waste mantle and any permeable rocks 
on which the mantle rests, as far as the first impermeable layer, 
where the descending movement of the water is stopped. The 
deep zones of flow 
occupy any pervi- 
ous rocks which 
may be found be- 
lt w the impervious 
layer which lies 
nearest to the sur- 




S 



FIG. 26. Geological Conditions favorable to 
Strong Springs 



a, limestone; 6, shale; c, coarse sandstone; d, lime- 
stone ; e, sandstone ; /, fissure. The strata dip toward 
the south, S. Redraw the diagram, marking the points 
at which strong springs (ss) may be expected 



face. The upper 

zone is a vast sheet 

of water saturating 

the soil and rocks and slowly seeping downward through their 

pores and interstices along the slopes to the valleys, where in 

part it discharges in springs and often unites also in a wide 

underflowing stream which supports and feeds the river (Fig. 24). 



FIG. 28 
FIG. 27 

FIG. 27. Diagram of Well which goes dry in Drought, a, and of Unfailing 

Well, b 

KM raw the diagram, showing by dotted line the normal ground-water surface 
and by broken line the ground-water surface at times of drought 

Ku;. 28. Diagram of Wet Weather Stream, a, and of Permanent Stream, 6 
Redraw the diagram, showing ground-water surface by dotted line 

A city in a region of copious rains, built on the narrow flood plain 
of a river, overlooked by hills, depends for its water supply on driven 
wells, within the city limits, sunk in the sand a few yards from the edge 
of the stream. Are these wells fed by water from the river percolating 
through the sand, or by ground water on its way to the stream and 
possibly contaminated with the sewage of the town ? 



44 THE ELEMENTS OF GEOLOGY 

At what height does underground water stand in the wells of your 
region ? Does it vary with the season ? Have you ever known wells to 
go dry ? It may be possible to get data from different wells and to draw 
a diagram showing the ground-water surface as compared with the sur- 
face of the ground. 

Fissure springs and artesian wells. The deeper zones of flow 
lie in pervious strata which are overlain by some impervious 
stratum. Such layers are often carried by their dip to great 
depths, and water may circulate in them to far below the level 
of the surface streams and even of the sea. When a fissure 
crosses a water-bearing stratum, or aquifer, water is forced 




FIG. 29. Section across South Dakota from the Black Hills to Sioux 
Falls (S), illustrating the Conditions of Artesian Wells 

a, crystalline impervious rocks; 6, sedimentary rocks, shales, limestones, 
and sandstones; c, pervious sandstone, the aquifer; d, impervious 
shales; w, w, w, artesian wells 

upward by the pressure of the weight of the water contained in 
the higher parts of the stratum, and may reach the surface as a 
fissure spring. A -boring which taps such an aquifer is known 
as an artesian well, a name derived from a province in France 
where wells of this kind have been long in use. The rise of 
the water in artesian wells, and in fissure springs also, depends 
on the folio wing conditions illustrated in Figure 29. The aquifer 
dips toward the region of the wells from higher ground, where 
it outcrops and receives its water. It is inclosed between an 
impervious layer above and water-tight or water-logged layers 
beneath. The weight of the column of water thus inclosed in 
the aquifer causes water to rise in the well, precisely as the 
weight of the water in a standpipe forces it in connected pipes 
to the upper stories of buildings. 



THE WORK OF GROUND WATER 45 

Which will supply the larger region with artesian wells, an aquifer 
whose dip is steep or one whose dip is gentle? W^hich of the two 
aquifers, their thickness being equal, will have the larger outcrop and 
therefore be able to draw upon the larger amount of water from the 
rainfall? Illustrate with diagrams. 

The zone of solution. Near the surface, where the circulation 
of ground water is most active, it oxidizes, corrodes, and dissolves 
the rocks through which it passes. It leaches soils and subsoils 
of their lime and other soluble minerals upon which plants 
depend for their food. It takes away the soluble cements of 
rocks ; it widens fissures and joints and opens winding passages 




FIG. 30. Diagram of Caverns and Sink Holes 

along the bedding planes; it may even remove whole beds of 
soluble rocks, such as rock salt, limestone, or gypsum. The 
work of ground water in producing landslides has already been 
noticed. The zone in which the work of ground water is thus 
for the most part destructive we may call the zone of solution. 
Caves. In massive limestone rocks, ground water dissolves 
channels which sometimes form large caves (Fig. 30). The 
necessary conditions for the excavation of caves of great size 
are -well shown in central Kentucky, where an upland is built 
throughout of thick horizontal beds of limestone. The absence 
of layers of insoluble or impervious rock in its structure allows 
a free circulation of ground water within it by the way of all 
natural openings in the rock. These water ways have been gradu- 
ally enlarged by solution and wear until the upland is honey- 
combed with caves. Five hundred open caverns are known in 
one county. 



46 



THE ELEMENTS OF GEOLOGY 



Mammoth Cave, the largest of these caverns, consists of a labyrinth 
of chambers and winding galleries whose total length is said to be 
as much as thirty miles. One passage four miles long has an average 
width of about sixty feet and an average height of forty feet. One of 
the great halls is three hundred feet in width and is overhung by a 
solid arch of limestone one hundred feet above the floor. Galleries at 
different levels are connected by well-like pits, some of which measure 
two hundred and twenty-five feet from top to bottom. Through some of 
the lowest of these tunnels flows Echo River, still at work dissolving 
and wearing away the rock while on its dark way to appear at the 
surface as a great spring. 

Natural bridges. As a cavern enlarges and the surface of the 
land above it is lowered by weathering, the roof at last breaks 
down and the cave becomes an open ravine. A portion of the 
roof may for a \vhile remain, forming a " natural bridge." 

Sink holes. In limestone regions channels under ground may 
become so well developed that the water of rains rapidly drains 

away through them. 
Ground w a t e r 
stands low and wells 
must be sunk deep 
to find it. Little or 
no surface water is 
left to form brooks. 



Thus across the 
limestone upland of 
central Kentucky one 
meets but three sur- 
face streams in a 




FIG. 31. Sink Holes in the Karst, Austria 



hundred miles. Between their valleys surface water finds its way under- 
ground by means of sink holes. These are pits, commonly funnel 
shaped, formed by the enlargement of crevice or joint by percolating 
water, or by the breakdown of some portion of the roof of a cave. By 
clogging of the outlet a sink hole may come to be filled by a pond. 

Central Florida is a limestone region with its drainage largely sub- 
terranean and in part below the level even of the sea. Sink holes are 



THE WORK OF GROUND WATER 



47 



common, and many of them are occupied by lakelets. Great springs 
mark the point of issue of underground streams, while some rise from 
beneath the sea. Silver Spring, one of the largest, discharges from a 
basin eight hundred feet wide and thirty feet deep a little river navi- 
gable for small steamers to its source. About the spring there are no 
surface streams for sixty miles. 

The Karst. Along the eastern coast of the Adriatic, as far south as 
Montenegro, lies a belt of limestone mountains singularly worn and 
honeycombed by the sol- 
vent action of water. 
Where forests have been 
cut from the mountain 
sides and the red soil has 
washed away, the surface 
of the white limestone 
forms a pathless desert of 
rock where each square rod 
has been corroded into an 
intricate branch work of 
shallow furrows and sharp 
ridges. Great sink holes, 
some of them six hundred 
feet deep and more, pock- 
mark the surface of the 
land. The drainage is 
chiefly subterranean. Sur- 
face streams are rare and 
a portion of their courses 
is often under ground. 
Fragmentary valleys come 
suddenly to an end at walls 
of rock where the rivers which occupy the valleys plunge into dark 
tunnels to reappear some miles away. Ground water stands so far 
below the surface that it cannot be reached by wells, and the inhabi- 
tants depend on rain water stored for household uses. The finest 
cavern of Europe, the Adelsberg Grotto, is in this region. Karst, the 
name of a part of this country, is now used to designate any region 
or landscape thus sculptured by the chemical action of surface and 
ground water. We must remember that Karst regions are rare, and 




FIG. 32. Underground Stream issuing from 
Base of Cliff, the Karst, Austria 



48 



THE ELEMENTS OF GEOLOGY 



striking as is the work of their subterranean streams, it is far less 
important than the work done by the sheets of underground water 
slowly seeping through all subsoils and porous rocks in other regions. 
Even when gathered into definite channels, ground water does not have 
the erosive power of surface streams, since it carries with it little or no 
rock waste. Regions whose underground drainage is so perfect that the 
development of surface streams has been retarded or prevented escape 
to a large extent the leveling action of surface running waters, and may 
therefore stand higher than the surrounding country. The hill honey- 
combed by Luray Cavern, Virginia, has been attributed to this cause. 

Cavern deposits. Even in the zone of solution water may under 
certain circumstances deposit as well as erode. As it trickles 

from the roof 
of caverns, the 
lime carbonate 
which it has 
taken into so- 
lution from the 
layers of lime- 
stone above is 
deposited by 
evaporation in 
the air in icicle- 
like pendants 
called stalac- 
tites. As the 
drops splash on 
the floor there 

are built up in the same way thicker masses called stalagmites, 
which may grow to join the stalactites above, forming pillars. 
A stalagmitic crust often seals with rock the earth which 
accumulates in caverns, together with whatever relics of cave 
dwellers, either animals or men, it may contain. 

Can you explain why slender stalactites formed by the drip of single 
drops are often hollow pipes ? 




FIG. 33. Stalactites and Stalagmites, Marengo 
Cavern, Indiana 



THE WORK OF GROUND WATER 49 

The zone of cementation. With increasing depth subterranean 
water becomes more and more sluggish in its movements and 
more and more highly charged with minerals dissolved from 
the rocks above. At such depths it deposits these minerals in 
the pores of rocks, cementing their grains together, and in 
crevices and fissures, forming mineral veins. Thus below the 
zone of solution where the work of water is to dissolve, lies the 
zone of cementation where its work is chemical deposit. A part 
of the invisible load of waste is thus transferred from rocks 
near the surface to those at greater depths. 

As the land surface is gradually lowered by weathering and 
the work of rain and streams, rocks which have lain deep within 
the zone of cementation are brought within the zone of solution. 
Tims' there are exposed to view limestones, whose cracks were 
filled with calcite (crystallized carbonate of lime), with quartz or 
other minerals, and sandstones whose grains were well cemented 
many feet below the surface. 

Cavity filling. Small cavities in the rocks are often found more or 
less completely filled with minerals deposited from solution by water in 
its constant circulation underground. The process may be illustrated 
by the deposit of salt crystals in a cup of evaporating brine, but in the 
latter instance the solution is not renewed as in the case of cavities in 
the rocks. A cavity thus lined with inward-pointing crystals is called 
a geode. 

Concretions. Ground water seeping through the pores of rocks may 
gather minerals disseminated throughout them into nodular masses 
called concretions. Thus silica disseminated through limestone is 
gathered into nodules of flint. While geodes grow from the outside 
inwards, concretions grow outwards from the center. Nor are they 
formed in already existing cavities as are geodes. In soft clays con- 
cretions may, as they grow, press the clay aside. In many other rooks 
concretions are made by the process of replacement. Molecule by mole- 
cule the rock is removed and the mineral of the concretion substituted 
in its place. The concretion may in this way preserve intact the lami- 
nation lines or other structures of the rock. Clays and shales often 



50 



THE ELEMENTS OF GEOLOGY 



contain concretions of lime carbonate, of iron carbonate, or of iron 
sulphide. Some fossil, such as a leaf or shell, frequently forms the 
nucleus around which the concretion grows. 

Why are building stones more easily worked when " green " than 
after their quarry water has dried out? 

Deposits of ground water in arid regions. In arid lands where 
ground water is drawn by capillarity to the surface and there 

evaporates, it leaves as 
surface incrustations the 
minerals held in solution. 
White limy incrusta- 
tions of this nature cover 
considerable tracts 
in northern Mexico. 
Evaporating beneath the 
surface, ground water 
may deposit a limy 
cement in beds of loose 
sand and gravel. Such 
firmly cemented layers 




FIG. 34. Concretions in Sandstone, 
Wyoming 



are not uncommon in western Kansas and Nebraska, where 
they are known as "mortar beds." 

Thermal springs. While the lower limit of surface drainage is 
sea level, subterranean water circulates much below that depth, 
and is brought again to the surface by hydrostatic pressure. In 
many instances springs have a higher temperature than the 
average annual temperature of the region, and are then known 
as thermal springs. In regions of present or recent volcanic 
activity, such as the Yellowstone National Park, we may believe 
that the heat of thermal springs is derived from uncooled lavas, 
perhaps not far below the 'surf ace. But when hot springs occur 
at a distance of hundreds of miles from any volcano, as in the 
case of the hot springs of Bath, England, it is probable that 
their waters have risen from the heated rocks of the earth's 



THE WORK OF GROUND WATER 



51 



interior. The springs of Bath have a temperature of 120 K, 
70 above the average annual temperature of the place. If we 
assume that the rate of increase in the earth's internal heat is 
here the average rate, 1 F. to every sixty feet of descent, we 
may conclude that the springs of Bath rise from at least a depth 
of forty-two hundred feet. 

Water may descend to depths from which it can never be 
brought back by hydrostatic pressure. It is absorbed by highly 
heated rocks deep below the surface. From time to time some 
of this deep-seated water may be returned to open air in the 
steam of volcanic eruptions. 




FIG. 35. Calcareous Deposits from Hot Springs, Yellowstone 
National Park 

Surface deposits of springs. Where subterranean water re- 
turns to the surface highly charged with minerals in solution, on 
exposure to the air it is commonly compelled to lay down much 
of its invisible load in chemical deposits about the spring. These 
are thrown down from solution either because of cooling, evap- 
oration, the loss of carbon dioxide, or the work of algae. 

Many springs have been charged under pressure with carbon 
dioxide from subterranean sources and are able therefore to 



52 THE ELEMENTS OF GEOLOGY 

take up large quantities of lime carbonate from the limestone 
rocks through which they pass. On. reaching the surface the 
pressure is relieved, the gas escapes, and the lime carbonate is 
thrown down in deposits called travertine. The gas is some- 
times withdrawn and the deposit produced in large part by the 
action of algae and other humble forms of plant life. 

At the Mammoth Hot Springs in the valley of the Gardiner River, 
Yellowstone National Park, beautiful terraces and basins of travertine 
(Fig. 35) are now building, chiefly by means of algae which cover the 
bottoms, rims, and sides of the basins and deposit lime carbonate upon 
them in successive sheets. The rock, snow-white where dry, is coated 
with red and orange gelatinous mats where the algse thrive in the over- 
flowing waters. 

Similar terraces of travertine are found to a height of fourteen 
hundred feet up the valley side. We may infer that the springs which 
formed these ancient deposits discharged near what was then the bottom 
of the valley, and that as the valley has been deepened by the river the 
ground water of the region has found lower and lower points of issue. 

In many parts of the country calcareous springs occur which coat 
with lime carbonate mosses, twigs, and other objects over which their 
waters flow. Such are popularly known as petrifying springs, although 
they merely incrust the objects and do not convert them into stone. 

Silica is soluble in alkaline waters, especially when these are 
hot. Hot springs rising through alkaline siliceous rocks, such as 
lavas, often deposit silica in a white spongy formation known 
as siliceous sinter, both by evaporation and by the action of 
algae which secrete silica from the waters. It is in this way that 
the cones and mounds of the geysers in the Yellowstone National 
Park and in Iceland have been formed (Fig. 234). 

Where water oozes from the earth one may sometimes see 
a rusty deposit on the ground, and perhaps an iridescent scum 
upon the water. The scum is often mistaken for oil, but at a 
touch it cracks and breaks, as oil would not do. It is a film of 
bydrated iron oxide, or limonite, and the spring is an iron, or 
chalybeate, spring. Compounds of iron have been taken into 



THE WORK OF GROUND WATER 53 

solution by ground water from soil and rocks, and are now 
changed to the insoluble oxide on exposure to the oxygen of 
the air. 

In wet ground iron compounds leached by ground water from the 
soil often collect in reddish deposits a few feet below the surface, where 
their downward progress is arrested by some impervious clay. At the 
bottom of bogs and shallow lakes iron ores sometimes accumulate to a 
depth of several feet. 

Decaying organic matter plays a large part in these changes. In its 
presence the insoluble iron oxides which give color to most red and 
yellow rocks are decomposed, leaving the rocks of a gray or bluish color, 
and the soluble iron compounds which result are readily leached out, 
effects seen where red or yellow clays have been bleached about some 
decaying tree root. 

The iron thus dissolved is laid down as limonite when oxidized, as 
about a chalybeate spring ; but out of contact with the air and in the 
presence of carbon dioxide supplied by decaying vegetation, as in a peat 
bog, it may be deposited as iron carbonate, or siderite. 

Total amount of underground waters. In order to realize 
the vast work in solution and cementation which underground 
waters are now doing and have done in all geological ages, 
we must gain some conception of their amount. At a certain 
depth, estimated at about six miles, the weight of the crust be- 
comes greater than the rocks can bear, and all cavities and pores 
in them must be completely closed by the enormous pressure 
which they sustain. Below a depth of even three or four miles 
it is believed that ground water cannot circulate. Estimating 
the average pore spaces of the different rocks of the earth's crust 
above this depth, and the average per cents of their pore spaces 
occupied by water, it has been recently computed that the total 
amount of ground water is equal to a sheet of water one hundred 
feet deep, covering the entire surface of the earth. 



CHAPTEE III 
RIVERS AND VALLEYS 

The run-off. We have traced the history of that portion of 
the rainfall which soaks into the ground ; let us now return to 
that part which washes along the surface and is known as the 
run-off. Fed by rains and melting snows, the run-off gathers 
into courses, perhaps but faintly marked at first, which join 
more definite and deeply cut channels, as twigs their stems. In 
a humid climate the larger ravines through which the run-off 
flows soon descend below the ground-water surface. Here 
springs discharge along the sides of the little valleys and per- 
manent streams begin. The water supplied by the run-off here 
joins that part of the rainfall which had soaked into the soil, 
and both now proceed together by way of the stream to the sea. 

River floods. Streams vary greatly in volume during the year. 
At stages of flood they fill their immediate banks, or overrun 
them and inundate any low lands adjacent to the channel ; at 
stages of low water they diminish to but a fraction of their vol- 
ume when at flood. 

At times of flood, rivers are fed chiefly by the run-off; at 
times of low water, largely or even wholly by springs. 

How, then, will the water of streams differ at these times in tur- 
bidity and in the relative amount of solids carried in solution? 

In parts of England streams have been known to continue flowing 
after eighteen months of local drought, so great is the volume of water 
which in humid climates is stored in the rocks above the drainage level, 
and so slowly is it given off in springs. 

In Illinois and the states adjacent, rivers remain low in winter and a 
" spring freshet " follows the melting of the winter's snows. A " June 

64 



RIVERS AND VALLEYS 55 

rise " is produced by the heavy rains of early summer. Low water fol- 
lows in July and August, and streams are again swollen to a moderate 
degree under the rains of autumn. 

The discharge of streams. The per cent of rainfall discharged 
by rivers varies with the amount of rainfall, the slope of the 
drainage area, the texture of the rocks, and other factors. With 
an annual rainfall of fifty inches hi an open country, about fifty 
per cent is discharged ; while with a rainfall of twenty inches 
only fifteen per cent is discharged, part of the remainder being 
evaporated and part passing underground beyond the drainage 
area. Thus the Ohio discharges thirty per cent of the rainfall of 
its basin, while the Missouri carries away but fifteen per cent. 
A number of the streams of the semi-arid lands of the West do 
not discharge more than five per cent of the rainfall. 

Other things being equal, which will afford the larger proportion of 
run-off, a region underlain with granite rock or with coarse sandstone ? 
grass land or forest ? steep slopes or level land ? a well-drained region 
or one abounding in marshes and ponds ? frozen or unfrozen ground ? 
Will there be a larger proportion of run-off after long rains or after 
a season of drought ? after long and gentle rains, or after the same 
amount of precipitation in a violent rain? during the months of grow- 
ing vegetation, from June to August, or during the autumn months? 

Desert streams. In arid regions the ground-water surface lies 
so low that for the most part stream ways do not intersect it. 
Streams therefore are not fed by springs, 
but instead lose volume as their waters 

soak into the thirsty rocks over which ' 

they flow. They contribute to the ground FIG. 36. Rise of Ground- 
water of the region instead of being in- Water Surface (broken 
. _ . . .. , , . _ , line) beneath Valley 

creased by it. Being supplied chiefly by (F) in Arid Region 

the run-off, they wither at times of drought 

to a mere trickle of water, to a chain of pools, or go wholly 
dry, while at long intervals rains fill their dusty beds with 
sudden raging torrents. Desert rivers therefore periodically 



56 THE ELEMENTS OF GEOLOGY 

shorten and lengthen their courses, withering back at times of 
drought for scores of miles, or even for a hundred miles from 
the point reached by their waters during seasons of rain. 

The geological work of streams. The work of streams is of 
three kinds, transportation, erosion, and deposition. Streams 
transport the waste of the land ; they wear, or erode, their chan- 
nels both on bed and banks; and they deposit portions of their 
load from time to time along their courses, finally laying it 
down in the sea. Most of the work of streams is done at times 
of flood. 

TRANSPORTATION 

The invisible load of streams. Of the waste which a river 
transports we may consider first the invisible load which it carries 
in solution, supplied chiefly by springs but also in part by the 
run-off and from the solution of the rocks of its bed. More 
than half the dissolved solids in the water of the average river 
consists of the carbonates of lime and magnesia ; other sub- 
stances are gypsum, sodium sulphate (Glauber's salts), mag- 
nesium sulphate (Epsom salts), sodium chloride (common salt), 
and even silica, the least soluble of the common rock-making 
minerals. The amount of this invisible load is surprisingly 
large. The Mississippi, for example, transports each year 113,- 
000,000 tons of dissolved rock to the Gulf. 

The visible load of streams. This consists of the silt which 
the stream carries in suspension, and the sand and gravel and 
larger stones which it pushes along its bed. Especially in times 
of flood one may note the muddy water, its silt being kept from 
settling by the rolling, eddying currents ; and often by placing 
his ear close to the bottom of a boat one may hear the clatter 
of pebbles as they are hurried along. In mountain torrents the 
rumble of bowlders as they clash together may be heard some 
distance away. The amount of the load which a stream can 
transport depends on its velocity. A current of two thirds of a 



RIVERS AND A' ALLEYS 57 

mile per hour can move fine sand, while one of four miles per 
hour sweeps along pebbles as large as hen's eggs. The trans- 
porting power of a stream varies as the sixth power of its velocity. 
If its velocity is multiplied by two, its transporting power is 
multiplied by the sixth power of two : it can now move stones 
sixty-four times as large as it could before. 

Stones weigh from two to three times as much as water, and in water 
lose the weight of the volume of water which they displace. What 
proportion, then, of their weight in air do stones lose when submerged ? 

Measurement of stream loads. To obtain the total amount of 
waste transported by a river is an important but difficult matter. 
The amount of water discharged must first be found by multi- 
plying the number of square feet in the average cross section of 
the stream by its velocity per second, giving the discharge per 
second in cubic feet. The amount of silt to a cubic foot of 
water is found by filtering samples of the water taken from 
different parts of the stream and at different times in the year, 
and drying and weighing the residues. The average amount of 
silt to the cubic foot of water, multiplied by the number of 
cubic feet of water discharged per year, gives the total load 
carried in suspension during that time. Adding to this the 
estimated amount of sand and gravel rolled along the bed, 
which in many swift rivers greatly exceeds the lighter material 
held in suspension, and adding also the total amount of dis- 
solved solids, we reach the exceedingly important result of the 
total load of waste discharged by the river. Dividing the 
volume of this load by the area of the river basin gives 
another result of the greatest geological interest, the rate at 
which the region is being lowered by the combined action of 
weathering and erosion, or the rate of denudation. 

The rate of denudation of river basins. This rate varies widely. 
The Mississippi basin may be taken as a representative land 
surface because of the varieties of surface, altitude and slope, 



58 THE ELEMENTS OF GEOLOGY 

climate, and underlying rocks which are included in its great 
extent. Careful measurements show that the Mississippi basin 
is now being lowered at a rate of one four-thousandth of a foot 
a year, or one foot in four thousand years. Taking this as the 
average rate of denudation for the land surfaces of the globe, 
estimates have been made of the length of time required at this 
rate to wash and wear the continents to the level of the sea. 
As the average elevation of the lands of the globe is reckoned 
at 2411 feet, this result would occur in nine or ten million 
years, if the present rate of denudation should remain unchanged. 
But even if no movements of the earth's crust should lift or 
depress the continents, the rate of wear and the removal of 
waste from their surfaces will not remain the same. It must 
constantly decrease as the lands are worn nearer to sea level 
and their slopes become more gentle. The length of time 
required to wear them away is therefore far in excess of that 
just stated. 

The drainage area of the Potomac is 11,000 square miles. The silt 
brought down in suspension in a year would cover a square mile to the 
depth of four feet. At what rate is the Potomac basin being lowered 
from this cause alone ? 

It is estimated that the Upper Ganges is lowering its basin at the 
rate of one foot in 823 years, and the Po one foot in 720 years. Why 
so much faster than the Potomac and the Mississippi? 

How streams get their loads. The load of streams is derived 
from a number of sources, the larger part being supplied by the 
weathering of valley slopes. We have noticed how the mantle 
of waste creeps and washes to the stream ways. Watching the 
run-off during a rain, as it hurries muddy with waste along the 
gutter or washes down the hillside, we may see the beginning 
of the route by which the larger part of their load is delivered 
to rivers. Streams also secure some of their load by wearing it 
from their beds and banks, a process called erosion. 



RIVERS AND VALLEYS 59 

EROSION 

Streams erode their beds chiefly by means of their bottom 
load, the stones of various sizes and the sand and even the fine 
mud which they sweep along. With these tools they smooth, 
grind, and rasp the rock of their beds, using them in much the 
fashion of sandpaper or a file. 

Weathering of river beds. The erosion of stream beds is 
greatly helped by the work of the weather. Especially at low 
water more or less of the bed is exposed to the action of frost and 
heat and cold, joints 
are opened, rocks 
are pried loose and 
broken up and made 
ready to be swept 
away by the stream 
at time of flood. 

Potholes. In 
rapids streams also 
drill out their rocky 
beds. Where some 

5 lg epressiOll F]( . 37 p thole in Bed of Stream, Ireland 

gives rise to an 

eddy, the pebbles which gather in it are whirled round and 
round, and, acting like the bit of an auger, bore out a cylin- 
drical pit called a pothole. Potholes sometimes reach a depth 
of a score of feet. Where they are numerous they aid mate- 
rially in deepening the channel, as the walls between them are 
worn away and they coalesce. 

Waterfalls. One of the most effective means of erosion which 
the river possesses is the waterfall. The plunging water dis- 
lodges stones from the face of the ledge over which it pours, 
and often undermines it by excavating a deep pit at its base. 
Slice after slice is thus thrown down from the front of the 




60 



THE ELEMENTS OF GEOLOGY 




FIG. 38. Map of the Gorge of the 
Niagara River 



cliff, and the cataract cuts 
its way upstream leaving a 
gorge behind it. 

Niagara Falls. The Niag- 
ara River flows from Lake 
Erie at Buffalo in a broad 
channel which it has cut but 
a few feet below the level of 
the region. Some thirteen 
miles from the outlet it 
plunges over a ledge one 
hundred and seventy feet 
high into the head of a nar- 
row gorge which extends for 
seven miles to the escarp- 
ment of the upland in which 
the gorge is cut. The strata 
which compose the upland 
dip gently upstream and con-' 
sist at top of a massive lime- 
stone, at the Falls about 
eighty feet thick, and below 
of soft and easily weathered 
shale. Beneath the Falls the 
underlying shale is cut and 
washed away by the descend- 
ing water and retreats also 
because of weathering, while 
the overhanging limestone 
breaks down in huge blocks 
from time to time. 



Niagara is divided by Goat Island into the Horseshoe Falls 
and the American Falls. The former is supplied by the main 
current of the river, and from the semicircular sweep of its 



RIVERS AND VALLEYS 61 

rim a sheet of water in places at least fifteen or twenty feet 
deep plunges into a pool a little less than two hundred feet in 
depth. Here the force of the falling water is sufficient to move 
about the fallen blocks of limestone and use them in the exca- 
vation of the shale of the bed. At the American Falls the 
lesser branch of the river, which flows along the American 
side of Goat Island, pours over the side of the gorge and breaks 
upon a high talus of limestone blocks which its smaller volume 
of water is unable to grind to pieces and remove. 

A series of surveys have determined that from 1842 to 1911 
the Horseshoe Falls retreated at the rate of about five feet a year, 
while the American Falls retreated at about one twentieth of 



R W EN 




012345 

FIG. 39. Longitudinal Section of Niagara Gorge 

Black, water; F, falls; R, rapids; W, whirlpool; E, escarpment; 
N, north ; S, south 

this rate. We cannot doubt that the same agency which is 
now lengthening the gorge at this rapid rate has cut it back 
its entire length of seven miles. 

While Niagara Falls have been cutting back a gorge seven 
miles long and from two hundred to three hundred feet deep, 
the river above the Falls has eroded its bed scarcely below the 
level of the upland on which it flows. Like all streams which 
are the outlets of lakes, the Niagara flows out of Lake Erie clear 
of sediment, as from a settling basin, and carries no tools with 
which to abrade its bed. We may infer from this instance how 
slight is the erosive power of clear water on hard rock. 

Assuming that the rate of recession of the combined volumes of the 
American and Horseshoe Falls was five feet a year below Goat Island 
and assuming that this rate has been uniform in the past, how long is it 
since the Niagara River fell over the edge of the escarpment where 
now is the mouth of the present gorge? 



62 THE ELEMENTS OF GEOLOGY 

The profile of the bed of the Niagara along the gorge (Fig. 39) shows 
alternating deeps and shallows which cannot be accounted for, except 
in a single instance, by the relative hardness of the rocks of the river 
bed. The deeps do not exceed that at the foot of the Horseshoe Falls 
at the present time. When the gorge was being cut along the shallows, 
how did the Falls compare in excavating power, in force, and volume 
with the Niagara of to-day? How did the rate of recession at those 
times compare with the present rate ? Is the assumption made above 
that the rate of recession has been uniform correct? 

The first stretch of shallows below the Falls causes a tumultuous 
rapid impossible to sound. Its depth has been estimated at thirty-five 
feet. From what data could such an estimate be made ? 

Suggest a reason why the Horseshoe Falls are convex upstream. 

At the present rate of recession which will reach the head of Goat 
Island the sooner, the American or the Horseshoe Falls? What will 
be the fate of the Falls left behind when the other has passed beyond 
the head of the island ? 

The rate at Avhich a stream erodes its bed depends in part upon the 
nature of the rocks over which it flows. Will a stream deepen its chan- 
nel more rapidly on massive or on thin-bedded and close-jointed rocks? 
on horizontal strata or on strata steeply inclined ? 

DEPOSITION 

While the river carries its invisible load of dissolved rock on 
without stop to the sea, its load of visible waste is subject to 
many delays en route. Now and again it is laid aside, to be 
picked up later and carried some distance farther on its way. 
One of the most striking features of the river therefore is the 
waste accumulated along its course, in bars and islands in the 
channel, beneath its bed, and in flood plains along its banks. 
All this alluvium, to use a general term for river deposits, 
with which the valley is cumbered is really en route to the sea ; 
it is only temporarily laid aside to resume its journey later on. 
Constantly the river is destroying and rebuilding its alluvial 
deposits, here cutting and there depositing along its banks, 
here eroding and there building a bar, here excavating its bed 



64 



THE ELEMENTS OF GEOLOGY 



and there filling it up, and at all times carrying the material 
picked up at one point some distance on downstream before 
depositing it at another. 

These deposits are laid down by slackening currents where 
the velocity of the stream is checked, as 011 the inner side of 
curves, and where the slope of the bed is diminished, and in the 
lee of islands, bridge piers and projecting points of land. How 
slight is the check required to cause a current to drop a large 




FIG. 41. Sand Bar deposited by Stream, showing Cross Bedding 

part of its load may be inferred from the law of the relation 
of the transporting power to the velocity. If the velocity is 
decreased one half, the current can move fragments but one 
sixty-fourth the size of those which it could move before, and 
must drop all those of larger size. 

Will a river deposit more at low water or at flood ? when rising or 
when falling? 

Stratification. Paver deposits are stratified, as may be seen in 
any fresh cut in banks or bars. The waste of which they are 



RIVERS AND VALLEYS 65 

luiilt has been sorted and deposited in layers, one above another ; 
some of finer and some of coarser material. The sorting action 
of running water depends on the fact that its transporting 
power varies with the velocity. A current whose diminishing 
velocity compels it to drop coarse gravel, for example, is still 
able to move all the finer waste of its load, and separating 
it from the gravel, carries it on downstream; while at a later 
time slower currents may deposit on the gravel bed layers of 
sand, and, still later, slack water may leave on these a layer of 
mud. In case of materials lighter than water the transporting 
power does not depend on the velocity, and logs of wood, for 
instance, are floated on to the sea on the slowest as well as on 
the most rapid currents. 

Cross bedding. A section of a bar exposed at low water may 
show that it is formed of layers of sand, or coarser stuff, inclined 
downstream as steeply often as the angle of repose of the 
material. From a M r _'... - 

CL ' /, 

boat anchored over " irvr "' "^ ^ 

the lower end of a FlG ' 42 ' Longitudinal Section of a River Bar 

submerged sand bar we may observe the way in which this 
structure, called cross bedding, is produced. Sand is continually 
pushed over the edge of the bar at b (Fig. 42) and comes to rest 
in successive layers on the sloping surface. At the same time 
the bar may be worn away at the upper end, a, and thus slowly 
advance down stream. While the deposit is thus cross bedded, 
it constitutes as a whole a stratum whose upper and lower 
surfaces are about horizontal. In sections of river banks one 
may often see a vertical succession of cross-bedded strata, each 
built in the way described. 

Water wear. The coarser material of river deposits, such as 
cobblestones, gravel, and the larger grains of sand, are water worn, 
or rounded, except when near their source. Rolling along the 
bottom they have been worn round by impact and friction as 
they rubbed against one another and the rocky bed of the stream. 



66 



THE ELEMENTS OF GEOLOGY 



Experiments have shown that angular fragments of granite lose 
nearly half their weight and become well rounded after traveling fif- 
teen miles in rotating cylinders partly filled with water. Marbles 
are cheaply made in Germany out of small limestone cubes set revolving 




FIG. 43. Water-Worn Pebbles, Upper Potomac River, Maryland 

in a current of water between a rotating bed of stone and a block 
of oak, the process requiring but about fifteen minutes. It has been 
found that in the upper reaches of mountain streams a descent of less 
than a mile is sufficient to round pebbles of granite. 



LAND FORMS DUE TO EIVER EROSION 

River valleys. In their courses to the sea, rivers follow val- 
leys of various forms, some shallow and some deep, some 
narrow and some wide. Since rivers are known to erode their 
beds and banks, it is a fair presumption that, aided by the 
weather, they have excavated the valleys in which. they now. 

Moreover, a bird's-eye view or a map of a region shows the 
significant fact that the valleys of a system unite with one 
another in a branch work, as twigs meet their stems and the 



RIVERS AND VALLEYS 67 

branches of a tree its trunk. Each valley, from that of the 
smallest rivulet to that of the master stream, is proportionate 
to the size of the stream which occupies it. With a few 
explainable exceptions the valleys of tributaries join that of 
the trunk stream at a level ; there is no sudden descent or 
break in the bed at the point of juncture. These are the 
natural consequences which must follow if the land has long 
been worked upon by streams, and no other process has ever 
been suggested which is competent to produce them. We must 
conclude that valley systems -have been formed by the river 
systems which drain them, aided by the work of the weather ; 
they are not gaping fissures in the earth's crust, as early ob- 
servers imagined, but are the furrows which running water has 
drawn upon the land. 

As valleys are made by the slow wear of streams and the 
action of the weather, they pass in their development through 
successive stages, each of which has its own characteristic 
features. We may therefore classify rivers and valleys accord- 
ing to the stage which they have reached in their life history 
from infancy to old age. 

Young River Valleys 

Infancy. The Red River of the North. A region in northwestern 
Minnesota and the adjacent portions of North Dakota and Manitoba was 
so recently covered by the waters of an extinct lake, known as Lake 
Agassiz, that the surface remains much as it was left when the lake 
was drained away. The flat floor, spread smooth with lake-laid silts, is 
still a plain, to the eye as level as the sea. Across it the Red River of 
the North and its branches run in narrow, ditch-like channels, steep- 
sided and shallow, not exceeding sixty feet in depth, their gradients 
differing little from the general slopes of the region. The trunk streams 
have but few tributaries ; the river system, like a sapling with few 
limbs, is still undeveloped. Along the banks of the trunk streams short 
gullies are slowly lengthening headwards, like growing twigs which 
are sometime to become large branches. 




68 



RIVERS AND VALLEYS 69 

The flat interstream areas are as yet but little scored by drainage 
lines, and in wet weather water lingers in ponds in any initial depres- 
sions on the plain. 

Contours. In order to read the topographic maps of the text-book and 
the laboratory the student should know that contours are lines drawn 
on maps to represent relief, all points on any given contour being of equal 
height above sea level. The contour interval is the uniform vertical 
distance between two adjacent contours and varies on different maps. 




FIG. 45. A Young River, Iowa 

Note that it has hardly begun to cut a valley in the plain of glacial 
drift on which it flows 

To express regions of faint relief a contour interval of ten or twenty 
feet is commonly selected; while in mountainous regions a contour 
interval of two hundred and fifty, five hundred, or even one thousand 
feet may be necessary in order that the contours may not be too 
crowded for easy reading. 

Whether a river begins its life on a lake plain, as in the 
example just cited, or upon a coastal plain lifted from beneath 
the sea or on a spread of glacial drift left by the retreat of 
continental ice sheets, such as covers much of Canada and the 
northeastern parts of the United States, its infantile stage pre- 
sents the same characteristic features, a narrow and shallow 
valley, with undeveloped tributaries and undrained interstream 
areas. Ground water stands high, and, exuding in the undrained 
initial depressions, forms marshes and lakes. 



70 



THE ELEMENTS OF GEOLOGY 



Lakes. Lakes are perhaps the most obvious of these fleeting 
features of infancy. They are short-lived, for their destruction 
is soon accomplished by several means. As a river system ad- 
vances toward maturity the deepening and extending valleys of 
the tributaries lower the ground-water surface and invade the 
undrained depressions of the region. Lakes having outlets are 




FIG. 46. A Young Drift Region in Wisconsin 

Describe this area. How high are the hills? Are they such in form and 
position as would be left by stream erosion ? Consult a map of the entire 
state and notice that the Fox River finds way to Lake Michigan, while 
the Wisconsin empties into the Mississippi. Describe that portion of the 
divide here shown between the Mississippi and the St. Lawrence systems. 
Which is the larger river, the Wisconsin or the Fox ? Other things being 
equal, which may be expected to deepen its bed the more rapidly ? 
What changes are likely to occur when one of these rivers comes to flow 
at a lower level than the other? Why have not these changes occurred 
already ? 

drained away as their basin rims are cut down by the outflow- 
ing streams, a slow process where the rim is of hard rock, 
but a rapid one where it is of soft material such as glacial drift. 
Lakes are effaced also by the filling of their basins. Inflow- 
ing streams and the wash of rains bring in waste. Waves abrade 
the shore and strew the debris worn from it over the lake bed. 
Shallow lakes are often filled with organic matter from decay- 
ing vegetation. 

Does the outflowing stream from a lake carry sediment? How does 
this fact affect its erosive power on hard rock ? on loose material ? 



RIVERS AND VALLEYS 



71 



Lake Geneva is a well-known example of a lake in process of obliter- 
ation. The inflowing Rhone has already displaced the waters of the 
lake for a length of twenty miles with the waste brought down from 
the high Alps. For this distance there extends up the Rhone Valley 
an alluvial plain, which has grown lakeward at the rate of a mile and 
a half since Roman times, as proved by the distance inland at which 
a Roman port now stands. 

How rapidly a lake may be silted up under exceptionally favorable 
conditions is illustrated by the fact that over the bottom of the artificial 
lake, of thirty-five square miles, formed 
behind the great dam across the Colorado 
River at Austin, Texas, sediments thirty- 
nine feet deep gathered in seven years. 

Lake Mendota, one of the many beauti- 
ful lakes of southern Wisconsin, is rapidly 
cutting back the soft glacial drift of its 
shores by means of the abrasion of its 
waves. While the shallow basin is thus 
broadened, it is also being filled with the 
waste ; and the time is brought nearer when it will be so shoaled that 
vegetation can complete the work of its effacement. 




A Small Lake being 
broadened and shoaled by 
Wave Wear 

Is, lake surface; dotted line, 
initial shore ; a, cut made by 
waves; 6, fill made of mate- 
rial taken from a 




FIG. 48. A Lake in Process of Effacement, Montana 
By what means is the lake bed being filled ? 



72 THE ELEMENTS OF GEOLOGY 

Along the margin of a shallow lake mosses, water lilies, 
grasses, and other water-loving plants grow luxuriantly. As 
their decaying remains accumulate on the bottom, the ring of 
marsh broadens inwards, the lake narrows gradually to a small 
pond set in the midst of a wide bog, and finally disappears. 
All stages in this process of extinction may be seen among 
the countless lakelets which occupy sags in the recent sheets 




FIG. 49. A Level Meadow, Scotland 
Explain its origin. What will be its future? 

of glacial drift in the northern states ; and more numerous than 
the lakes which still remain are those already thus filled with 
carbonaceous matter derived from the carbon dioxide of the 
atmosphere. Such fossil lakes are marked by swamps or level 
meadows underlain with muck. 

The advance to maturity. The infantile stage is brief. As 
a river advances toward maturity the initial depressions, the 
lake basins of its area, are gradually effaced. By the furrowing 
action of the rain wash and the headward lengthening of tribu- 
taries a branchwork of drainage channels grows until it covers 
the entire area, and not an acre is left on which the fallen 



RIVERS AND VALLEYS 



73 



raindrop does not find already cut for it an uninterrupted down- 
ward path which leads it on by way of gully, brook, and river 
to the sea. The initial surface of the land, by whatever agency 




FIG. 50. Drainage Maps 

A, an area in its infancy, Buena Vista County, Iowa; B, an area in its 
maturity, Riuggold County, Iowa 

it was modeled, is now wholly destroyed ; the region is all 
reduced to valley slopes. 

The longitudinal profile of a stream. This at first corresponds 
with the initial surface of the region on which the stream 
begins to flow, although its way may lead through basins and 
down steep descents. 

The successive pro- ^ \ v 

files to which it re- 
duces its bed are 
illustrated in Fig- 
ure 51. As the gra- 
dient, or rate of de- 
scent of its bed, is 
lowered, the veloc- 
ity of the river is 
decreased until its 




FIG. 51. Successive Longitudinal Profiles 
of a Stream 

am, initial profile, with waterfall at w, and basins at I 
and I', which at first are occupied by lakes and 
later are filled or drained ; 6, c, d, and e, profiles 
established in succession as the stream advances 
from infancy toward old age. Note that these 
profiles are concave toward the sky. This is the 
erosion curve. What contrasting form has the 
weather curve (p. 34) ? 



74 THE ELEMENTS OF GEOLOGY 

lessening energy is wholly consumed in carrying its load and it 
can no longer erode its bed. The river is now at grade, and its 
capacity is just equal to its load. If now its load is increased 
the stream deposits, and thus builds up, or aggrades, its bed. 
On the other hand, if its load is diminished it has energy to 
spare, and resuming its w^ork of erosion, degrades its bed. In 
either case the stream continues aggrading or degrading until 




FIG. 52. A V-Valley, the Canyon of the Yellowstone 

Note the steep sides. What processes are at work upon them ? How wide 
is the valley at base compared with the width of the stream ? Do you 
see any river deposits along its banks? Is the stream flowing swiftly 
over a rock bed, or quietly over a bed which it has built up? Is it 
graded or ungraded? Note that the canyon walls project in interlock- 
ing spurs 

a new gradient is found where the velocity is just sufficient to 
move the load, and here again it reaches grade. 

V-Valleys. Vigorous rivers well armed with waste make short 
work of cutting their beds to grade, and thus erode narrow, 
steep-sided gorges only wide enough at the base to accommodate 
the stream. The steepness of the valley slopes depends on the 
relative rates at which the bed is cut down by the stream and 
the sides are worn back by the weather. In resistant rock a 



RIVERS AND VALLEYS 



75 



swift, well-laden stream may saw out a gorge whose sides are 
nearly or even quite vertical, but as a rule young valleys 
whose streams have not yet reached grade are V-shaped ; their 
sides flare at the top because here the rocks have longest been 
opened up to the action of the weather. Some of the deepest 
canyons may be found where a 
rising land mass, either mountain 
range or plateau, has long main- 
tained by its continued uplift the 
rivers of the region above grade. 

In the northern hemisphere the north FlG - 
sides of river valleys are sometimes of 




500 1000 1500 



FEET 



Neglecting any cutting of the 
river against its banks, estimate 
what part of the excavation of 
the canyon is due to the vertical 
erosion of its bed by the river 
and what to weathering and 
rain wash on the canyon sides 



Section of the Yellow- 
more gentle slope than the south sides. This canvon is 100 feet dee P' 2 500 
feet wide at the top, and about 

Can you suggest a reason? m feet wide at tbe bottom 

The Grand Canyon of the Colorado 
River in Arizona. The Colorado River 
trenches the high plateau of northern 
Arizona with a colossal canyon two 
hundred and eighteen miles long and 
more than a mile in greatest depth 
(Fig. 15). The rocks in which the canyon is cut are for the most part 
flat-lying, massive beds of limestones and sandstones, with some shales, 
beneath which in places harder crystalline rocks are disclosed. Where 
the canyon is deepest its walls have been profoundly dissected. Lateral 
ravines have widened into immense amphitheaters, leaving between 
them long ridges of mountain height, buttressed and rebuttressed with 
flanking spurs and carved into majestic architectural forms. From the 
extremity of one of these promontories it is two miles or more across 
the gulf to the point of the one opposite, and the heads of the amphi- 
theaters are thirteen miles apart. 

The lower portion of the canyon is much narrower (Fig. 54) and its 
svalls of dark crystalline rock sink steeply to the edge of the river, a 
swift, powerful stream a few hundred feet wide, turbid with reddish 
silt, by means of which it continually rasps its rocky bed as it hurries 
on. The Colorado is still deepening its gorge. In the Grand Canyon 
its gradient is seven and one half feet to the mile, but, as in all 
ungraded rivers, the descent is far from uniform. Graded reaches in 




76 



RIVERS AND VALLEYS 77 

soft rock alternate with steeper declivities in hard rock, forming rapids 
such as, for example, a stretch of ten miles where the fall averages 
twenty-one feet to the mile. Because of these dangerous rapids the few 
exploring parties who have traversed the Colorado canyon have done so 
at the hazard of their lives. 

The canyon has been shaped by several agencies. Its depth is due 
to the river which has sawed its way far toward the base of a lofty 
rising plateau. Acting alone this would have produced a slitlike gorge 
little wider than the breadth of the stream. The impressive width of 
the canyon and the magnificent architectural masses which fill it are 
owing to two causes. Running water has gulched the walls and 
weathering has everywhere attacked and driven them back. The hori- 
zontal harder beds stand out in long lines of vertical cliffs, often hun- 
dreds of feet in height, at whose feet talus slopes conceal the outcrop 




B 
FIG. 66. Diagrams illustrating Conditions which produce Falls or Rapids 

A, vertical succession of harder and softer rocks ; B, horizontal succession of the 
same. In A the stream ab in sinking its bed through a mass of strata of dif- 
ferent degrees of hardness has discovered the weak layer s beneath the hard 
layer h. It rapidly cuts its way in *, while in h its work is delayed. Thus 
the profile a/6' is soon reached, with falls at/. In B the initial profile is 
shown by dotted line. 

of the weaker strata (Fig. 15). As the upper cliffs have been sapped 
and driven back by the weather, broad platforms are left at their bases 
and the sides of the canyon descend to the river by gigantic steps. Far 
up and down the canyon the eye traces these horizontal layers, like the 
flutings of an elaborate molding, distinguishing each by its contour as 
well as by its color and thickness. 

The Grand Canyon of the Colorado is often and rightly cited as an 
example of the stupendous erosion which may be accomplished by a 
river. And yet the Colorado is a young stream and its work is no more 
than well begun. It has not yet wholly reached grade, and the great 
task of the river and its tributaries the task of leveling the lofty 
plateau to a low plain and of transporting it grain by grain to the sea 
still lies almost entirely in the future. 



78 



THE ELEMENTS OF GEOLOGY 




lv tnf 



!c 



Waterfalls and rapids. Before the bed of a stream is reduced 
to grade it may be broken by abrupt descents which give rise to 
waterfalls and rapids. Such breaks in a river's bed may belong 

to the initial surface over which 
it began its course ; still more 
commonly are they developed 
in the rock mass through which 
it is cutting its valley. Thus, 
wherever a stream leaves harder 
rocks to flow over softer ones the 

to, lavas deeply decayed through latter are quickly worn below the 
action of thermal waters ; m and level of the former, and a sharp 

m', masses of undecayed lavas to i -\ -.T p -n 

whose hardness the falls are dne. dian g e m sl P e > Wlth a Waterfall 

Which fall will be worn away the or rap^d, results (Fig. 55). 
sooner? How far upstream will 



FIG. 56. Longitudinal Section of 
Yellowstone River at Lower 
Fall, F, and Upper Fall, F', 
Yellowstone National Park 



each fall migrate? Draw profile 
of the river when one fall has dis- 
appeared 




At time of flood young tributaries 
with- steeper courses than that of the 
trunk stream may bring down stone? 

and finer waste, which the gentler current cannot move along, ana 
throw them as a dam across its way. The rapids thus formed are also 
ephemeral, for as the gradient 
of the tributaries is lowered the 
main stream becomes able to 
handle the smaller and finer 
load which they discharge. 

A rare class of falls is pro- 
duced where the minor tribu- 
taries of a young river are not 
able to keep pace with their 
master stream in the erosion 
of their beds because of their 
smaller volume, and thus join 
it by plunging over the side 



FIG. 57. Diagram illustrating Migration 
of a Fall due to a Hard Layer 77, in 
the Midst of Soft Layers S and S, all 
dipping upstream 

a, b t c, d, and e, successive profiles of the 
stream ; /, /', and /", successive posi- 
tions of the fall ; r, rapid to which the 
fall is reduced. Draw diagram showing 
migration of fall in strata dipping down- 
stream. Under what conditions of incli- 
nation of the strata will a fall migrate 
the farthest and have the longest life? 
Under what conditions will it migrate the 
least distance and soonest be destroyed ? 



of its gorge. But as the river 

approaches grade and slackens 

its down cutting, the tributaries sooner or later overtake it, and, 

effacing their falls, unite with it on a level. 




Contour Interval 100 feet 



FIG. 58. Maturely Dissected Plateau near Charleston, West Virginia 

Compare the number of streams in any given number of square miles 
with the number on an area of the same size in the Red River valley 
(Fig. 44). What is the shape of the ridges? Are their summits broad 
or narrow ? Are their crests even or broken by knobs and cols (the 
depressions on the crest line) ? If the latter, how deeply have the cols 
been worn beneath the summits of the knobs ? 
79 



80 THE ELEMENTS OF GEOLOGY 

Waterfalls and rapids of all kinds are evanescent features of 
a river's youth. Like lakes they are soon destroyed, and if any 
long time had already elapsed since their formation they would 
have been obliterated already. 

Local baselevels. That balanced condition called grade, where 
a river neither degrades its bed by erosion nor aggrades it by 
deposition, is first attained along reaches of soft rocks, ungraded 
outcrops of hard rocks remaining as barriers which give rise to 
rapids or falls. Until these barriers are worn away they con- 
stitute local baselevels, below which level the stream, up valley 




FIG. 59. A Maturely Dissected Region of Slight Relief, Iowa 

from them, cannot cut. They are eroded to grade one after 
another, beginning with the least strong, or the one nearest 
the mouth of the stream. In a similar way the surface of a 
lake in a river's course constitutes for all inflowing streams 
a local baselevel, which disappears when the basin is filled or 
drained. 

Mature and Old Rivers 

Maturity is the stage of a river's complete development and 
most effective work. The river system now has well under way 
its great task of wearing down the land mass which it drains 
and carrying it particle by particle to the sea. The relief of the 
land is now at its greatest ; for the main channels have been 



RIVERS AND VALLEYS 



81 



sunk to grade, while 
the divides remain 
but little worn below 
their initial altitudes. 
Ground water now 
stands low. The run- 
off washes directly to 
the streams, w r ith the 
least delay and loss 
by evaporation in 
ponds and marshes; 
the discharge of the 
river is therefore at 
its height. The entire 
region is dissected by 
stream ways. The 
area of valley slopes 
is now largest and 
sheds to the streams 
a heavier load of 
waste than ever be- 
fore. At maturity the 
river system is doing 
its greatest amount of 
work both in erosion 
and in the carriage 
of water and of waste 
to the sea. 

Lateral erosion. 
On reaching grade a 
river ceases to scour 
its bed, and it does 
not again begin to do 
so until some change 




. 

II 



>> 73 






C 2? "* 

> co 

c? "C^ 



5- 

^09 






^ ll 



82 THE ELEMENTS OF GEOLOGY 

in load or volume enables it to find grade at a lower level. On the 
other hand, a stream erodes its banks at all stages in its history, 
and with graded rivers this process, called lateral erosion, or 
planation, is specially important. The current of a stream fol- 
lows the outer side of all curves or bends in the channel, and on 
this side it excavates its bed the deepest and continually wears 
and saps its banks. On the inner side deposition takes place in 
the more shallow and slower-moving water. The inner bank of 
bends is thus built out while the outer bank is worn away. By 
swinging its curves against the valley sides a graded river con- 
tinually cuts a wider and wider floor. The V-valley of youth is 
thus changed by planation to a flat-floored valley with flaring 
sides which gradually become subdued by the weather to gentle 
slopes. While widening their valleys streams maintain a con- 
stant width of channel, so that a wide-floored valley does not 
signify that it ever was occupied by a river of equal width. 

The gradient. The gradients of graded rivers differ widely. 
A large river with a light load reaches grade on a faint slope, 
while a smaller stream heavily burdened with waste requires 
a steep slope to give it velocity sufficient to move the load. 

The Platte, a graded river of Nebraska with its headwaters in the 
Rocky Mountains, is enfeebled by the semi-arid climate of the Great 
Plains and surcharged with the waste brought down both by its branches 
in the mountains and by those whose tracks lie over the soft rocks 
of the plains. It is compelled to maintain a gradient of eight feet to 
the mile in western Nebraska. The Ohio reaches grade with a slope 
of less than four inches to the mile from Cincinnati to its mouth, and 
the powerful Mississippi washes along its load with a fall of but three 
inches per mile from Cairo to the Gulf. 

Other things being equal, which of graded streams will have the 
steeper gradient, a trunk stream or its tributaries ? a stream supplied 
with gravel or one with silt ? 

Other factors remaining the same, what changes would occur if the 
Platte should increase in volume ? What changes would occur if the load 
should be increased in amount or in coarseness? 



RIVERS AND VALLEYS 



83 



The old age of rivers. As rivers pass their prime, as denuda- 
tion lowers the relief of the region, less waste and finer is 
washed over the gentler slopes of the lowering hills. With 
smaller loads to carry, the rivers now deepen their valleys and 
find grade with fainter declivities nearer the level of the sea. 
This limit of the level of the sea beneath which they cannot 
erode is known as baselevel. 1 As streams grow old they approach 
more and more closely to baselevel, although they are never 
able to attain it. Some slight slope is needed that \vater may 
flow and waste be transported over the land. Meanwhile the 




FIG. 61. Successive Cross Sections of a Region as it advances 
from Infancy a, to Old Age e 

relief of the land has ever lessened. The master streams and 
their main tributaries now wander with sluggish currents over 
the broad valley floors which they have planed away; while 
under the erosion of their innumerable branches and the wear 
of the weather the divides everywhere are lowered and subdued 
to more and more gentle slopes. Mountains and high plateaus 
are thus reduced to rolling hills, and at last to plains, sur- 
mounted only by such hills as may still be unreduced to the 
common level, because of the harder rocks of which they are 
composed or because of their distance from the main erosion 
channels. Such regions of faint relief, worn down to near base 
level by subaerial agencies, are known as peneplains (almost 
plains). Any residual masses which rise above them are called 
monadnocks, from the name of a conical peak of New Hampshire 
which overlooks the now uplifted peneplain of southern New 
England. 

1 The term "baselevel" is also used to designate the close approximation to 
sea level to which streams are able to subdue the land. 



84 THE ELEMENTS OF GEOLOGY 

In its old age a region becomes mantled with thick sheets 
of fine and weathered waste, slowly moving over the faint slopes 
toward the water ways and unbroken by ledges of bare rock. 
In other words, the waste mantle also is now graded, and as 
waterfalls have been effaced in the river beds, so now any 
ledges in the wide streams of waste are worn away and covered 
beneath smooth slopes of fine soil. Ground water stands high 
and may exude in areas of swamp. In youth the land mass 
was roughhewn and cut deep by stream erosion. In old age 




Fi<;. (52. Peneplain surmounted by Monadnocks, Piedmont 
Belt, Virginia 

From Davis' Elementary Physical Geography 

the faint reliefs of the land dissolve away, chiefly under the 
action of the weather, beneath their cloak of waste. 

The cycle of erosion. The successive stages through which 
a land mass passes while it is being leveled to the sea consti- 
tute together a cycle of erosion. Each stage of the cycle from 
infancy to old age leaves, as we have seen, its characteristic 
records in the forms sculptured on the land, such as the shapes 
of valleys and the contours of hills and plains. The geologist 
is thus able to determine by the land forms of any region the 
stage in the erosion cycle to which it now belongs, and know- 
ing what are the earlier stages of the cycle, to read something 
of the geological history of the region. 



RIVERS AND VALLEYS 



85 



Interrupted cycles. So long a time is needed to reduce a land 
mass to baselevel that the process is seldom if ever completed 
during a single uninterrupted cycle of erosion. Of all the vari- 
ous interruptions which may occur the most important are 
gradual movements of the earth's crust, by which a region is 
either depressed or elevated relative to sea level. 

The depression of a region hastens its old age by decreasing 
the gradient of streams, by destroying their power to excavate 
their beds and cany their loads to a degree corresponding to 




FIG. 63. Young Inner Gorge in Wide Older Valley, Alaska 

the amount of the depression, and by lessening the amount of 
work they have to do. The slackened river currents deposit 
their waste in flood plains which increase in height as the sub- 
sidence continues. The lower courses of the rivers are invaded 
by the sea and become estuaries, while the lower tributaries are 
cut off from the trunk stream. 

Elevation, on the other hand, increases the activity of all 
agencies of weathering, erosion, and transportation, restores the 
region to its youth, and inaugurates a new cycle of erosion. 
Streams are given a steeper gradient, greater velocity, and 
increased energy to carry their loads and wear their beds. 



86 



THE ELEMENTS OF GEOLOGY 



They cut through the alluvium of their flood plains, leaving it 
on either bank as successive terraces, and intrench themselves 
hi the underlying rock. In their older and wider valleys they 
cut narrow, steep-walled inner gorges, in which they flow swiftly 
over rocky floors, broken here and there by falls and rapids 

where a harder layer of rock 
has been discovered. Wind- 
ing streams on plains may 
thus incise their meanders in 
solid rock as the plains are 
gradually uplifted. Streams 
which are thus restored to 
their youth are said to be 
revived. 

As streams cut deeper and 
the valley slopes are steep- 
ened, the mantle of waste of 
the region undergoing eleva- 
tion is set in more rapid 
movement. It is now re- 
moved particle by particle 
faster than it forms. As the 
waste mantle thins, weather- 
ing attacks the rocks of the 
region more energetically 
until an equilibrium is 
reached again; the rocks 
waste rapidly and their waste is as rapidly removed. 

Dissected peneplains. When a rise of the land brings one cycle 
to an end and begins another, the characteristic land forms of 
each cycle are found together and the topography of the region 
is composite until the second cycle is so far advanced that the 
land forms of the first cycle are entirely destroyed. The contrast 
between the land surfaces of the later and the earlier cycles is 




Contour Interval 50 feet 



FIG. 64. Incised Meanders of Oneota 
River, Iowa 



RIVERS AND VALLEYS 



8T 




most striking when the earlier had advanced to age and the 

later is still in youth. Thus many peneplains which have been 

elevated and dissected have been recognized by the remnants 

of their ancient erosion 

surfaces, and the length 

of time which has 

elapsed since their uplift 

has been measured by 

the stage to which the 

new cycle has advanced. 

The Piedmont Belt. As 
an example of an ancient 
peneplain uplifted and 
dissected we may cite the 
Piedmont Belt, a broad 
upland lying between the 
Appalachian Mountains 
and the Atlantic coastal 
plain. The surface of the 
Piedmont is gently rolling. 
The divides, which are 
often smooth areas of con- 
siderable width, rise to a 
common plane, and from 
them one sees in every 
direction an even sky line 
except where in places 
some lone hill or ridge may 
lift itself above the general 
level (Fig. 62). The sur- 
face is an ancient one, for 
the mantle of residual waste lies deep upon it, soils are reddened by 
long oxidation, and the rocks are rotted to a depth of scores of feet. 

At present, however, the waste mantle is not forming so rapidly as 
it is being removed. The streams of the upland are actively engaged in 
its destruction. They flow swiftly in narrow, rock-walled valleys over 
rocky beds. -This contrast between the young streams and the aged 



FIG. 65 

Describe the valley of stream a. Is it young or 
old ? How does the valley of b differ from that 
of a? Compare as to form and age the inner 
valley of & with the outer valley and with the 
valley of a. Account for the inner valley. 
Why does it not extend to the upper portion 
of the course of 6 ? Will it ever do so ? Draw 
longitudinal profile of b, showing the different 
gradient of upper and lower portions shown 
in diagram. We may suppose that a also has 
an inner valley in the lower portions of its 
course not here seen. As the inner valley of 
tributary c extends headward it may invade 
the valley of a before the inner valley of a 
has worked upstream to the area seen in the 
diagram. With what results ? 



88 



THE ELEMENTS OF GEOLOGY 



surface which they are now so vigorously dissecting can only be 
explained by the theory that the region once stood lower than at 
present and has recently been upraised. If now we imagine the valleys 
refilled with the waste which the streams have swept away, and the 




FIG. 66. Dissected Peneplain of Southern New England 

upland lowered, we restore the Piedmont region to the condition in 
which it stood before its uplift and dissection, a gently rolling plain, 
surmounted here and there by isolated hills and ridges. 

The surface of the ancient Piedmont plain, as it may be restored 
from the remnants of it found on the divides, is not in accordance with 
the structures of the country rocks. Where these are exposed to view 
they are seen to be far from horizontal. On the walls of river gorges 
they dip steeply and in various directions and the streams flow over 

M 



FIG. 07. Section in Piedmont Belt 
M, a monadnock 

their upturned edges. As shown in Figure 67, the rocks of the Piedmont 
have been folded and broken and tilted. 

It is not reasonable to believe that when the rocks of the Piedmont 
were thus folded and otherwise deformed the surface of the region was 
a plain. The upturned layers have not always stopped abruptly at the 
even surface of the Piedmont plain which now cuts across them. They 
are the bases of great folds and tilted blocks which must once have 



RIVERS AND VALLEYS 



89 



risen high in' air. The complex and disorderly structures of the Pied- 
mont rocks are those seen in great mountain ranges, and there is every 
reason to believe that these rocks after their deformation rose to moun- 
tain height. 

The ancient Piedmont plain cuts across these upturned rocks as 
independently of their structure as the even surface of the sawed stump 
of some great tree is independent of the direction of its fibers. Hence' 




FIG. 68. The Area of the Laurentian Peneplain (shaded) 

the Piedmont plain as it was before its uplift was not a coastal plain 
formed of strata spread in horizontal sheets beneath the sea and then 
uplifted ; nor was it a structural plain, due to the resistance to erosion 
of some hard, flat-lying layer of rock. Even surfaces developed on 
rocks of discordant structure, such as the Piedmont shows, are produced 
by long denudation, and we may consider the Piedmont as a peneplain 
formed by the wearing down of mountain ranges, and recently uplifted. 

The Laurentian peneplain. This is the name given to a 
denuded surface on very ancient rocks which extends from the 



90 THE ELEMENTS OF GEOLOGY 

Arctic Ocean to the St. Lawrence Eiver and Lake Superior, with 
small areas also in northern Wisconsin and New York. Through- 
out this U-shaped area, which incloses Hudson Bay within its 
arms, the country rocks have the complicated and contorted struc- 
tures which characterize mountain ranges (see Fig. 179, p. 211). 
But the surface of the area is by 110 means mountainous. The 
sky line when viewed from the divides is unbroken by moun- 
tain peaks or rugged hills. The surface of the arm west of 
Hudson Bay is gently undulating and that of the eastern arm 
lias been roughened to low-rolling hills and dissected in places 
by such deep river gorges as those of the Ottawa and Saguenay. 
This immense area may be regarded as an ancient peneplain 
truncating the bases of long-vanished mountains and dissected 
after elevation. 

In the examples cited the uplift has been a broad one and to 
comparatively little height. Where peneplains have been uplifted 
to great height and have since been well dissected, and where 
they have been upfolded and broken and uptilted, their recog- 
nition becomes more difficult. Yet recent observers have found 
evidences of ancient lowland surfaces of erosion on the summits 
of the Allegheny ridges, the Cascade Mountains (Fig. 69), and 
the western slope of the Sierra Nevadas. 

The southern Appalachian region. We have here an example of an 
area the latter part of whose geological history may be deciphered by 
means of its land forms. The generalized section of Figure 70, which 
passes from west to east across a portion of the region in eastern Ten- 
nessee, shows on the west a part of the broad Cumberland plateau. On 
the east is a roughened upland platform, from which rise in the distance 
the peaks of the Great Smoky Mountains. The plateau, consisting of 
strata but little changed from their original flat-lying attitude, and the 
platform, developed on rocks of disordered structure made crystalline 
by heat and pressure, both stand at the common level of the line 
ab. They are separated by the Appalachian valley, forty miles wide, 
cut in strata which have been folded and broken into long narrow 
blocks. The valley is traversed lengthwise by long, low ridges, the 



92 THE ELEMENTS OF GEOLOGY 

outcropping edges of the harder strata, which rise to about the same 
level, that of the line cd. Between these ridges stretch valley low- 
lands at the level ef, excavated in the weaker rocks, while somewhat 
below them lie the channels of the present streams now busily engaged 
in deepening their beds. 

The valley lowlands. Were they planed by graded or ungraded 
streams? Have the present streams reached grade? Why did the 
streams cease widening the floors of the valley lowlands? How long 




FIG. 70. Generalized Section of the Southern Appalachian Region in 
Eastern Tennessee 

since? When will they begin anew the work of lateral planation? 
What effect will this have on the ridges if the present cycle of erosion 
continues long uninterrupted? 

The ridges of the Appalachian valley. Why do they stand above the 
valley lowlands ? Why do their summits lie in about the same plane ? 
Refilling the valleys intervening between these ridges with the material 
removed by the streams, what is the nature of the surface thus restored ? 
Does this surface cd accord with the rock structures on which it 
has been developed ? How may it have been made ? At what height 
did the land stand then, compared with its present height ? What eleva- 
tions stood above the surface cdl Why? What name may you use to 
designate them ? How does the length of time needed to develop the 
surface cd compare with that needed to develop the valley lowlands? 

The platform and plateau. Why do they stand at a common level 
ab ? Of what surface may they be remnants ? Is it accordant with 
the rock structure ? How was it produced ? What unconsumed masses 
overlooked it? Did the rocks of the Appalachian valley stand above 
this surface when it was produced? Did they then stand below it? 
Compare the time needed to develop this surface with that needed to 
develop cd. Which surface is the older? 

. How many cycles of erosion are represented here ? Give the erosion 
history of the region by cycles, beginning with the oldest, the work done 
in each and the work left undone, what brought each cycle to a close, 
and how long relatively it continued. 



CHAPTER IV 
RIVER DEPOSITS 

The characteristic features of river deposits and the forms 
which they assume may be treated under three heads : (1) valley 
deposits, (2) basin deposits, and (3) deltas. 

VALLEY DEPOSITS 

Flood plains are the surfaces of the alluvial deposits which 
streams build along their courses at times of flood. A swift 
current then sweeps along the channel, while a shallow sheet 
of water moves slowly over the flood plain, spreading upon it a 
thin layer of sediment. It has been estimated that each inun- 
dation of the Nile leaves a layer of fertilizing silt three hun- 
dredths of an inch thick over the flood plain of Egypt. 

Flood plains may consist of a thin spread of alluvium over 
the flat rock floor of a valley which is being widened by the 
lateral erosion of a graded stream (Fig. 60). Flood-plain de- 
posits of great thickness may be built by 
aggrading rivers even in valleys whose 
rock floors have never been thus widened 
fFiff 368") FlGl ^* Cross Section of 

a Flood Plain 

A cross section of a flood plain (Fig. 71) 

shows that it is highest next the river, sloping gradually thence 
to the valley sides. These wide natural embankments are due 
to the fact that the river deposit is heavier near the bank, where 
the velocity of the silt-laden channel current is first checked by 
contact with the slower-moving overflow. 

93 



94 



THE ELEMENTS OF GEOLOGY 



Thus banked off from the stream, the outer portions of a 
flood plain are often ill-drained and swampy, and here vegetal 

deposits, such as peat, may 
be interbedded with river 

silts. 

A map of a wide flood plain, 
such as that of the Mississippi 
or the Missouri (Fig. 77), shows 
that the courses of the tribu- 
taries on entering it are de- 
flected downstream. Why? 

The aggrading streams 
by which flood plains are 
constructed gradually build 
their immediate banks and 
beds to higher and higher 
levels, and therefore find it 
easy at times of great floods 
to break their natural em- 
bankments and take new 
courses over the plain. In 
this way they aggrade each 
portion of it in turn by 
means of their shifting 
channels. 

Braided channels. A 
river actively engaged in 
aggrading its valley with 
coarse waste builds a flood 




plain of comparatively steep 



FIG. 72. Waste-Filled Valley and Braided 
Channels of the Upper Mississippi 

gradient and often flows down it in a fairly direct course and 
through a network of braided channels. From time to time 
a channel becomes choked with waste, and the water no longer 
finding room in it breaks out and cuts and builds itself a new 



RIVER DEPOSITS 



95 




FIG. 73. Terraced Valley of River in Central Asia 

way which reunites down valley with the other channels. Thus 
there becomes established a network of ever-changing channels 
inclosing low islands of sand and gravel 




FIG. 74. Terraces carved in Alluvial Deposits 
Modified after Davis 

Which is the older, the rock floor of the valley or the river deposits which 
fill it? "What are the relative ages of terraces a, 6, a, and e? It will 
be noted that the remnants of the higher flood plains have not been 
swept away by the meandering river, as it swung from side to side of 
the valley at lower levels, because they have been defended by ledges 
of hard rock in the projecting spurs of the initial valley. The stream 
has encountered such defending ledges at the points marked d 



96 



THE ELEMENTS OF GEOLOGY 



Terrace^. While aggrading streams thus tend to shift their 
channels, degrading streams, on the contrary, become more and 
more deeply intrenched in their valleys. It often occurs that a 

stream, after having 
built a flood plain, 
ceases to aggrade its bed 
because of a lessened 




FIG. 75. River Terraces of Rock covered 
with Alluvium 



load or for other reasons, 
such as an uplift of the 

c, recent flood plain of the river. To what pro- i v 

cesses is it due? Account for the alluvium at ie g lon > c 

a and b and on the opposite side of the valley at stead to degrade it. It 

the same levels. Which is the older? Account T ,*, -, a -, 

for the flat rock floors' on which these deposits leaves the Original flood 

of alluvium rest. Give the entire history which plain Ollt of reach of 
may be read in the section. ., , . , 

even the highest floods. 

When again it reaches grade at a lower level it produces a new 
flood plain by lateral erosion in the older deposits, remnants of 
which stand as terraces on one or both sides of the valley. In 
this way a valley may be lined with a succession of terraces at 
different levels, each level representing an abandoned flood plain. 
Meanders. Valleys aggraded with fine waste form well-nigh 
level plains over which streams wind from side to side of a 
direct course in sym- 
metric bends known 
as meanders, from the 
name of a winding 
river of Asia Minor. 
The giant Mississippi 





FIG. 76. Development of a Meander 



has developed mean- The dotted line in a, b, and c shows the stage pre- 



ders with a radius of 



ceding that indicated by the unbroken line 



one and one half miles, but a little creek may display on its 
meadow as perfect curves only a rod or so in radius. On the 
flood plain of either river or creek we may find examples of the 
successive stages in the development of the meander, from its 
beginning in the slight initial bend sufficient to deflect the 



RIVER DEPOSITS 



97 



current against the 
outer side. Eroding 
here and depositing 
on the inner side of 
the bend, it gradually 
reaches first the 
open bend (Fig. 7 6, a) 
whose width and 
length are not far 
from equal, and later 
that of the horseshoe 
meander (Fig. 76, V) 
whose diameter 
transverse to the 
course of the stream 
is much greater than 
that parallel with it. 
Little by little the 
neck of land project- 
ing into the bend is 
narrowed, until at 
last it is cut through 
and a "cut-off" is 
established. The old 
channel is now silted 




FIG. 77. Map of a Portion of the Flood Plain of 
the Missouri River 



up at both ends and Each gmall square represents one square mile. How 



becomes a crescentic 
lagoon (Fig. 76, c), 
or oxbow lake, which 
fills gradually to an 
arc-shaped shallow 
depression. 

Flood plains characteristic of mature rivers. On reaching 
grade a stream planes a flat floor for its continually widening 



wide is the flood plain of the Missouri ? How wide 
is the flood plain of the Big Sioux? Why is the 
latter river deflected down valley on entering the 
flood plain of its master stream ? How do the mean- 
ders of the two rivers compare in size ? How does 
the width of each flood plain compare with the 
width of the belt occupied hy the meanders of the 
river? Do you find traces of any former channels? 



98 



THE ELEMENTS OF GEOLOGY 



valley. Ever cutting 011 the outer bank of its curves, it deposits 
on the inner bank scroll-like flood-plain patches (Fig. 60). For a 
while the valley bluffs do not give its growing meanders room 
to develop to their normal size, but as planation goes on, the 
bluffs are driven back to the full width of the meander belt 
and still later to a width which gives room for broad stretches 
of flood plain on either side (Fig. 77). 

Usually a river first attains grade near its mouth, and here 
first sinks its bed to near baselevel. Extending its graded 

course upstream by 
cutting away bar- 
rier after barrier, it 
comes to have a 
widened and mature 
valley over its lower 
course, while its 
young headwaters 
are still busily erod- 
ing their beds. Its 
ungraded branches 
may thus bring 
down to its lower 
course more waste 
than it is competent to carry on to the sea, and here it aggrades 
its bed and builds a flood plain in order to gain a steeper gra- 
dient and velocity enough to transport its load. 

As maturity is past and the relief of the land is lessened, a 
smaller and smaller load of waste is delivered to the river. It 
now has energy to spare and again degrades its valley, excavat- 
ing its former flood plains and leaving them in terraces on either 
side, and at last in its old age sweeping them away. 

Alluvial cones and fans. In hilly and mountainous countries 
one often sees on a valley side a conical or fan-shaped deposit of 
waste at the mouth of a lateral stream. The cause is obvious : 




Fio. 78. Alluvial Cones, Wyoming 



RIVER DEPOSITS 



99 



the young branch has not been able as yet to wear its bed to 
accordant level with the already deepened valley of the master 
stream. It therefore builds its bed to grade at the point of junc- 
ture by depositing here its load of waste, a load too heavy to 
be carried along the more 
gentle profile of the trunk 
valley. 

Where rivers descend from a 
mountainous region upon the 
plain they may build alluvial 
fans of exceedingly gentle slope. 
Thus the rivers of the west- 
ern side of the Sierra Nevada 
Mountains have spread fans 
with a radius of as much as 
forty miles and a slope too 
slight to be detected without 
instruments, where they leave 
the rock-cut canyons in the 
mountains and descend upon 
the broad central valley of 
California. 

As a river flows over 
its fan it commonly divides 
into a branehwork of shift- 
ing channels called distribii- 
taries, since they lead off the 
water from the main stream. 




FIG. 79. Tributaries and Distributaries 
of a Fan-Building Stream 



In this way each part of the fan is aggraded and its symmetric 
form is preserved. 

Piedmont plains. Mountain streams may build their confluent 
fans into widespread piedmont (foot of the mountain) alluvial 
plains. These are especially characteristic of arid lands, where 
the streams wither as they flow out upon the thirsty lowlands 
and are therefore compelled to lay down a large portion of their 



100 THE ELEMENTS OF GEOLOGY 

load. In humid climates mountain-born streams are usually 
competent to carry their loads of waste on to the sea, and have 
energy to spare to cut the lower mountain slopes into foothills. 
In arid regions foothills are commonly absent and the ranges 
rise, as from pedestals, above broad, sloping plains of stream-laid 
waste. 

The High Plains. The rivers which flow eastward from the Rocky 
Mountains have united their fans in a continuous sheet of waste which 
stretches forward from the base of the mountains for hundreds of miles 
and in places is five hundred feet thick (Fig. 80). That the deposit was 
made in ancient times on land and not in the sea is proved by the 




FIG. 80. Section from the Rocky Mountains Eastward 
River deposits dotted 

remains which it contains of land animals and plants of species now 
extinct. That it was laid by rivers and not by fresh-water lakes is shown 
by its structure. Wide stretches of flat-lying clays and sands are inter- 
rupted by long, narrow belts of gravel which mark the channels of the 
ancient streams. Gravels and sands are often cross bedded, and their 
well-worn pebbles may be identified with the rocks of the mountains. 
After building this sheet of waste the streams ceased to aggrade and 
began the work of destruction. Large uneroded remnants, their sur- 
faces flat as a floor, remain as the High Plains of western Kansas and 
Nebraska. 

River deposits in subsiding troughs. To a geologist the most 
important river deposits are those which gather in areas of 
gradual subsidence ; they are often of vast extent and immense 
thickness, and such deposits of past geological ages have not 
infrequently been preserved, with all their records of the times 
in which they were built, by being carried below the level of 
the sea, to be brought to light by a later uplift. On the other 
hand, river deposits which remain above baselevels of erosion 
are swept away comparatively soon. 



RIVER DEPOSITS 



The Great Valley of California is a monotonously level plain of great 
fertility, four hundred miles in length and fifty miles in average width, 
built of waste swept down by streams from the mountain ranges which 
inclose it, the Sierra Nevada on the east and the Coast Range on the 
west. On the waste slopes at the foot of the bordering hills coarse 
gravels and even bowlders are left, while over the interior the slow- 
flowing streams at times of flood spread wide sheets of silt. Organic 
deposits are now forming by the decay of vegetation in swampy tule 
(reed) lands and in shallow lakes which occupy depressions left by the 
aggrading streams. 

Deep borings show that this great trough is rilled to a depth of at 
least two thousand feet below sea level with recent un consolidated 
sands and silts containing logs of wood and fresh-water shells. These 
are land deposits, and the absence of any marine deposits among them 
proves that the region has not been invaded by the sea since the 
accumulation began. It has therefore been slowly subsiding and its 
streams, although continually carried below grade, have yet been able 
to aggrade the surface as rapidly as the region sank, and have main- 
tained it, as at present, slightly above sea level. 

The Indo-Gangetic Plain, spread by the Brahmaputra, the Ganges, and 
the Indus river systems, stretches for sixteen hundred miles along the 
southern base of the Himalaya Mountains and occupies an area of 
three hundred thousand square miles (Fig. 342). It consists of the 
flood plains of the master streams and the confluent fans of the tribu- 
taries which issue from the mountains on the north. Large areas are 
subject to overflow each season of flood, and still larger tracts mark 
abandoned flood plains below which the rivers have now cut their beds. 
The plain is built of far-stretching beds of clay, penetrated by streaks 
of sand, and also of gravel near the mountains. Beds of impure peat 
occur in it, and it contains fresh-water shells and the bones of land 
animals of species now living in northern India. At Lucknow an 
artesian well was sunk to one thousand feet below sea level without 
reaching the bottom of these river-laid sands and silts, proving a slow 
subsidence with which the aggrading rivers have kept pace. 

Warped valleys. It is not necessary that an area should sink 
below sea level in order to be filled with stream-swept waste. 
High valleys among growing mountain ranges may suffer 
warping, or may be blockaded by rising mountain folds 



THE ELEMENTS OF GEOLOGY 

athwart them. Where the deformation is rapid enough, the 
river may be ponded and the valley filled with lake-laid sedi- 
ments. Even when the river is able to maintain its right of 
way it may yet have its declivity so lessened that it is com- 
pelled to aggrade its course continually, filling the valley with 
river deposits which may grow to an enormous thickness. 

Behind the outer ranges of the Himalaya Mountains lie several waste- 
filled valleys, the largest of which are Kashmir and Nepal, the former 
being an alluvial plain about as large as the state of Delaware. The 
rivers which drain these plains have already cut down their outlet 
gorges sufficiently to begin the task of the removal of the broad accu- 
mulations which they have brought in from the surrounding mountains. 
Their present flood plains lie as much as some hundreds of feet below 
wide alluvial terraces which mark their former levels. Indeed, the 
horizontal beds of the Hundes Valley have been trenched to the depth 
of nearly three thousand feet by the Sutlej River. These deposits are 
recent or subrecent, for there have been found at various levels the 
remains of land plants and land and fresh-water shells, and in some 
the bones of such animals as the hyena and the goat, of species or of 
genera now living. Such soft deposits cannot be expected to endure 
through any considerable length of future time the rapid erosion to 
which their great height above the level of the sea will subject them. 

Characteristics of river deposits. The examples just cited 
teach clearly the characteristic features of extensive river de- 

posits. These deposits 
consist of broad, flat- 
lying sheets of clay 
FIG. 81. Cross Section of Aggraded Valley, show- and fine sand left by 
ing Structure of River Deposits ^ overflow 




flood, and traversed here and there by long, narrow strips of 
coarse, cross-bedded sands and gravels thrown down by the 
swifter currents of the shifting channels. Occasional beds of 
muck mark the sites of shallow lakelets or fresh-water swamps. 
The various strata also contain some remains of the countless 
myriads of animals and plants which live upon the surface of 



RIVER DEPOSITS 103 

the plain as it is in process of building. River shells such as 
the mussel, land shells such as those of snails, the bones of 
tishes and of such land animals as suffer drowning at times 
of flood or are mired in swampy places, logs of wood, and the 
stems and leaves of plants are examples of the variety of the 
remains of land and fresh-water organisms which are entombed 
in river deposits and sealed away as a record of the life of the 
time, and as proof that the deposits were laid by streams and 
not beneath the sea. 

BASIN DEPOSITS 

Deposits in dry basins. On desert areas without outlet to the 
sea, as on the Great Basin of the United States and the deserts 
of central Asia, stream-swept waste accumulates indefinitely. 
The rivers of the surrounding mountains, fed by the rains and 
melting snows of these comparatively moist elevations, dry and 
soak away as they come down upon the arid plains. They are 
compelled to lay aside their entire load of waste eroded from 
the mountain valleys, in fans which grow to enormous size, 
reaching in some instances thousands of feet in thickness. 

The monotonous levels of Turkestan include vast alluvial tracts now 
in process of building by the floods of the frequently shifting channels 
of the Oxus and other rivers of the region. For about seven hundred 
miles from its mouth in Aral Lake the Oxus receives no tributaries, 
since even the larger branches of its system are lost in a network of dis- 
tributaries and choked with desert sands before they reach their master 
stream. These aggrading rivers, which have channels but no valleys, 
spread their muddy floods which in the case of the Oxus sometimes 
equal the average volume of the Mississippi far and wide over the 
plain, washing the bases of the desert dunes. 

Play as. In arid ulterior basins the central depressions may 
be occupied by playas, plains of fine mud washed forward 
from the margins. In the wet season the playa is covered with 
a thin sheet of muddy water, a playa lake, supplied usually by 



104 THE ELEMENTS OF GEOLOGY 

some stream at flood. In the dry season the lake evaporates, the 
river which fed it retreats, and there is left to view a hard, 
smooth, level floor of sun-baked and sun-cracked yellow clay 
utterly devoid of vegetation. 

In the Black Rock desert of Nevada a playa lake spreads over an 
area fifty miles long and twenty miles wide. In summer it disappears ; 
the Quinn River, which feeds it, shrinks back one hundred miles toward 
its source, leaving an absolutely barren floor of clay, level as the sea. 

Lake deposits. Regarding lakes as parts of river systems, 
we may now notice the characteristic features of the deposits in 
lake basins. Soundings in lakes of considerable size and depth 
show that their bottoms are being covered with fine clays. Sand 
and gravel are found along their margins, being brought in by 
streams and worn by waves from the shore, but there are no 
tidal or other strong currents to sweep coarse waste out from 
shore to any considerable distance. Where fine clays are now 
found on the land in even, horizontal layers containing the 
remains of fresh-water animals and plants, uncut by channels 
filled with cross-bedded gravels and sands and bordered by 
beach deposits of coarse waste, we may safely infer the exist- 
ence of ancient lakes. 

Marl. Marl is a soft, whitish deposit of carbonate of lime, mingled 
often with more or less of clay, accumulated in small lakes whose feed- 
ing springs are charged with carbonate of lime and into which little 
waste is washed from the land. Such lakelets are not infrequent on the 
surface of the younger drift sheets of Michigan and northern Indiana, 
where their beds of marl sometimes as much as forty feet thick are 
utilized in the manufacture of Portland cement. The deposit results 
from the decay of certain aquatic plants which secrete lime carbonate 
from the water, from the decomposition of the calcareous shells of tiny 
mollusks which live in countless numbers on the lake floor, and in some 
cases apparently from chemical precipitation. 

Peat. We have seen how lakelets are extinguished by the 
decaying remains of the vegetation which they support. A 



RIVER DEPOSITS 105 

section of such a fossil lake shows that below the growing 
mosses and other plants of the surface of the bog lies a spongy 
mass composed of dead vegetable tissue, which passes downward 
gradually into peat, a dense, dark brown carbonaceous deposit 
in which, to the unaided eye, little or no trace of vegetable 




FIG. 82. Digging Peat, Scotland 

structure remains. When dried, peat forms a fuel of some value 
and is used either cut into slabs and dried or pressed into bricks 
by machinery. , 

When vegetation decays in open air the carbon of its tissues, 
taken from the atmosphere by the leaves, is oxidized and re- 
turned to it in its original form of carbon dioxide. But decom- 
posing hi the presence of water, as in a bog, where the oxygen 
of the air is excluded, the carbonaceous matter of plants accumu- 
lates in deposits of peat. 

Peat bogs are numerous in regions lately abandoned by glacier ice, 
where river systems are so immature that the initial depressions left in 
the sheet of drift spread over the country have not yet been drained. 
One tenth of the surface of Ireland is said to be covered with peat, and 



106 THE ELEMENTS OF GEOLOGY 

small bogs abound in the drift-covered area of New England and the 
states lying a8 far west as the Missouri River. In Massachusetts alone it 
has been reckoned that there are fifteen billion cubic feet of peat, the 
largest bog occupying several thousand acres. 

Much larger swamps occur on the young coastal plain of the Atlantic 
from New Jersey to Florida. The Dismal Swamp, for example, in 
Virginia and North Carolina is forty miles across. It is covered with a 
dense growth of water-loving trees such as the cypress and black gum. 
The center of the swamp is occupied by Lake Drummond, a shallow 
lake seven miles in diameter, with banks of pure peat, and still narrow- 
ing from the encroachment of vegetation along its borders. 

Salt lakes. In arid climates a lake rarely receives sufficient 
inflow to enable it to rise to the basin rim and find an outlet. 
Before this height is reached its surface becomes large enough 
to discharge by evaporation into the dry air the amount of 
water that is supplied by streams. As such a lake has no out- 
let, the minerals in solution brought into it by its streams 
cannot escape from the basin. The lake water becomes more 
and more heavily charged with such substances as common 
salt and the sulphates and carbonates of lime, of soda, and of 
potash, and these are thrown down from solution one after 
another as the point of saturation for each mineral is reached. 
Carbonate of lime, the least soluble and often the most abundant 
mineral brought in, is the first to be precipitated. As concen- 
tration goes on, gypsum, which is insoluble in a strong brine, 
is deposited, and afterwards common salt. As the saltness of 
the lake varies with the seasons and with climatic changes, 
gypsum and salt are laid in alternate beds and are interleaved 
with sedimentary clays spread from the waste brought in by 
streams at times of flood. Few forms of life can live in bodies 
of salt water so concentrated that chemical deposits take place, 
and hence the beds of salt, gypsum, and silt of such lakes are 
quite barren of the remains of life. Similar deposits are pre- 
cipitated by the concentration of sea water in lagoons and arms 
of the sea cut off from the ocean. 



RIVER DEPOSITS 



107 



Lakes Bonneville and Lahontan. These names are given to extinct 
lakes which once occupied large areas in the Great Basin, the former 
in Utah, the latter in northwestern Nevada. Their records remain in 
various old beach lines which they drew along their mountainous shores 




* 



FIG. 83. Map of Lakes Bonneville and Lahontan 
From Davis' Physical Geography 

at the different levels at which they stood, and in the deposits of their 
beds. At its highest stage Lake Bonneville, then one thousand feet 
deep, overflowed to the north and was a fresh-water lake. As it shrank 
below the outlet it became more and more salty, and the Great Salt 
Lake, its withered residue, is now depositing salt along its shores. In 
its strong brine lime carbonate 
is insoluble, and that brought al 
in by streams is thrown down b\ 
at once in the form of traver- 
tine. 

Lake Lahontan never had an 
outlet. The first chemical de- 




FIG. 84. Section of Deposits in Beds of 
Lakes Bonneville and Lahontan 



posits to be made along its shores were deposits of travertine, in places 
eighty feet thick. Its floor is spread with fine clays, which must have 
been laid in deep, still water, and which are charged with the salts 



108 THE ELEMENTS OF GEOLOGY 

absorbed by them as the briny water of the lake dried away. These 
sedimentary clays are in two divisions, the upper and lower, each being 
about one hundred feet thick (a and c, Fig. 84). They are separated 
by heavy deposits of well-rounded, cross-bedded gravels and sands 
(6, Fig. 84), similar to those spread at the present time by the inter- 
mittent streams of arid regions. A similar record is shown in the old 
floors of Lake Bonneville. What conclusions do you draw from these 
facts as to the history of these ancient lakes? 

DELTAS 

In the river deposits which are left above sea level particles 
of waste are allowed to linger only for a time. From alluvial fans 
and flood plains they are constantly being taken up and swept 
farther on downstream. Although these land forms may long 
persist, the particles which compose them are ever changing. 
We may therefore think of the alluvial deposits of a valley as a 
stream of waste fed by the waste mantle as it creeps and washes 
down the valley sides, and slowly moving onwards to the sea. 

In basins waste finds a longer rest, but sooner or later lakes 
and dry basins are drained or filled, and their deposits, if 
above sea level, resume their journey to their final goal. It is 
only when carried below the level of the sea that they are 
indefinitely preserved. 

On reaching this terminus, rivers deliver their load to the 
ocean. In some cases the ocean is able to take it up by means 
of strong tidal and other currents, and to dispose of it in ways 
which we shall study later. But often the load is so large, or 
the tides are so weak, that much of the waste which the river 
brings in settles at its mouth, there building up a deposit called 
the delta, from the Greek letter (A) of that name, whose shape it 
sometimes resembles. 

Deltas and alluvial fans have many common characteristics. 
Both owe their origin to a sudden check in the velocity of the 
river, compelling a deposit of the load ; both are triangular in 



RIVER DEPOSITS 



109 



outline, the apex pointing upstream ; and both are traversed 
by distributaries which build up all parts in turn. 

In a delta we may distinguish deposits of two distinct kinds, 
the submarine and the subaerial In part a delta is built of 
waste brought down by the river and redistributed and spread 
by waves and tides over the sea bottom adjacent to the river's 
mouth. The origin of these deposits is recorded in the remains 
of marine animals and 
plants which they con- 
tain. 

As the submarine 
delta grows near to the 
level of the sea the dis- 
tributaries of the river 
cover it with subaerial 
deposits altogether 
similar to those of the 
flood plain, -of which 
indeed the subaerial 
delta is the prolongation. 




FIG. 85. Delta of the Mississippi River 

Here extended deposits of peat may 
accumulate in swamps, and the remains of land and fresh-water 
animals and plants swept down by the stream are imbedded in 
the silts laid at times of flood. 

Borings made in the deltas of great rivers such as the Missis- 
sippi, the Ganges, and the Nile, show that the subaerial portion 
often reaches a surprising thickness. Layers of peat, old soils, 
and forest grounds with the stumps of trees are discovered 
hundreds of feet below sea level In the Nile delta some eight 
layers of coarse gravel were found interbedded with river silts, 
and in the Ganges delta at Calcutta a boring nearly five hun- 
dred feet in depth stopped in such a layer. 

The Mississippi has built a delta of twelve thousand three hundred 
square miles, and is pushing the natural embankments of its chief dis- 
tributaries into the Gulf at a maximum rate of a mile in sixteen years. 



110 THE ELEMENTS OF GEOLOGY 

Muddy shoals surround its front, shallow lakes, e.g. lakes Pontchart- 
rain and Borgne, are formed between the growing delta and the old 
shore line, and elongate lakes and swamps are inclosed between the 
natural embankments of the distributaries. 

The delta of the Indus River, India, lies so low along shore that a 
broad tract of country is overflowed by the highest tides. The sub- 
marine portion of the delta has been built to near sea level over so wide 
a belt offshore that in many places large vessels cannot come even 
within sight of land because of the shallow water. 

A former arm of the sea, the Rann of Cutch, adjoining the delta on 
the east has been silted up and is now an immense barren flat of sandy 
mud two hundred miles in length and one hundred miles in greatest 
breadth. Each summer it is flooded with salt water when the sea is 
brought in by strong southwesterly monsoon winds, and the climate 
during the remainder of the year is hot and dry. By the evaporation 
of sea water the soil is thus left so salty that no vegetation can grow 

upon it, and in places beds 

^^^Bssss^-r ___ of salt several feet in thick- 



... :'-/> 



ness have accumulated. 
Under like conditions salt 

FIG. 86. Radial Section of a Delta beds of S reat thickness 

have been formed in the 
This section of a delta illustrates the structure of . 

the platform which swift streams well loaded P as ^ and are now ound 
with coarse waste build in the water bodies buried among the deposits 
into which they empty. Three members may o f ancient deltas, 
be distinguished : the bottom set beds, a ; the 
fore set beds, b; and the top set beds, c. 

Account for the slope of each of these. Why Subsidence Of great 

are the bottom set beds of the finer material 

and why do they extend beyond the others? deltas. As a rule great 
How does the profile of this delta differ from deltas ai'6 slowly sink- 
that of an alluvial cone, and why? 

ing. In some instances 

upbuilding by river deposits lias gone on as rapidly as the 
region has subsided. The entire thickness of the Ganges delta, 
for example, so far as it has been sounded, consists of deposits 
laid in open air. In other cases interbedded limestones and 
other sedimentary rocks containing marine fossils prove that at 
times subsidence has gained on the upbuilding and the delta 
has been covered with the sea. 



RIVER DEPOSITS 111 

It is by gradual depression that delta deposits attain enor- 
mous thickness, and, being lowered beneath the level of the 
sea, are safely preserved from erosion until a movement of the 
earth's crust in the opposite direction lifts them to form part of 
the land. We shall read later in the hard rocks of our continent 
the records of such ancient deltas, and we shall not be sur- 
prised to find them as thick as are those now building at the 
mouths of great rivers. 

Lake deltas. Deltas are also formed where streams lose their 
velocity on entering the still waters of lakes. The shore lines 
of extinct lakes, such as Lake Agassiz and Lakes Bonneville 
and Lahontan, may be traced by the heavy deposits at the 
mouths of their tributary streams. 



We have seen that the work of streams is to drain the lands 
of the water poured upon them by the rainfall, to wear them 
down, and to carry their waste away to the sea, there to be 
rebuilt by other agents into sedimentary rocks. The ancient 
strata of which the continents are largely made are composed 
chiefly of material thus worn from still more ancient lands 
lands with their hills and valleys like those of to-day and 
carried by their rivers to the ocean. In all geological times, as 
at the present, the work of streams has been to destroy the 
lands, and in so doing to furnish to the ocean the materials 
from which the lands of future ages were to be made. Before 
we consider how the waste of the land brought in by streams 
is rebuilt upon the ocean floor, we must proceed to study the 
work of two agents, glacier ice and the wind, which cooperate 
with rivers in the denudation of the land. 




112 



CHAPTEE V 
THE WORK OF GLACIERS 

The drift. The surface of northeastern North America, as far 
south as the Ohio and Missouri rivers, is generally covered by 
the drift, a formation which is quite unlike any which we 
have so far studied. A section of it, such as that illustrated in 
Figure 8 7, shows that for the most part it is unstratified, consisting 
of clay, sand, pebbles, and even large bowlders, all mingled pell- 
mell together. The agent which laid the drift is one which can 
carry a load of material of all sizes, from the largest bowlder to 
the finest clay, and deposit it without sorting. 

The stones of the drift are of many kinds. The region from 
which it was gathered may well have been large in order to 




FIG. 88. Characteristic Pebbles from the Drift 

No. 1 has six facets; Xo. 4, originally a rounded river pebble, has been 
rubbed down to one flat face ; Nos. 3 and 5 are battered subangular 
fragments faceted on one side only 

supply these many different varieties of rocks. Pebbles and 
bowlders have been left far from their original homes, as may 
be seen in southern Iowa, where the drift contains nuggets of 

113 



114 



THE ELEMENTS OF GEOLOGY 



copper brought from the region about Lake Superior. The 
agent which laid the drift is one able to gather its load over a 
large area and carry it a long way. 

The pebbles of the drift are unlike those rounded by running 
water or by waves. They are marked with scratches. Some 

are angular, many 
have had their edges 
blunted, while others 
have been ground 
flat and smooth 011 
one or more sides, 
like gems which 
have been faceted 
by being held firmly 
against the lapi- 
dary's wheel (Fig. 
88). In many places 
the upper surface of 
the country rock 
beneath the drift 
has been swept 
clean of residual 
clays and other 
waste. All rotten 
rock has been planed 
away, and the ledges 
of sound rock to which the surface has been cut down have 
been rubbed smooth and scratched with long, straight, parallel 
lines (Fig. 89). The agent which laid the drift can hold sand 
and pebbles firmly in its grasp and can grind them against the 
rock beneath, thus planing it down and scoring it, while faceting 
the pebbles also. 

Neither water nor wind can do these things. Indeed, noth- 
ing like the drift is being formed by any process now at work 




FIG. 89. Smoothed and Scored Rock Surface ex- 
posed to View by the Removal of Overlying 
Drift, Iowa 



THE WORK OF GLACIERS 



115 



anywhere in the eastern United States. To find the agent which 
has laid this extensive formation we must go to a region of 
different climatic con- 
ditions. 

The inland ice of 
Greenland. Green- 
land is about fifteen 
hundred miles long 
and nearly seven hun- 
dred miles in greatest 
width. With the ex- 
ception of a narrow 
fringe of mountainous 
coast land, it is com- 
pletely buried beneath 
a sheet of ice, in shape 
like a vast white 
shield, whose convex 
surface rises to a 
height of nine thou- 
sand feet above the 
sea. The few explor- 
ers who have crossed 
the ice cap found it a 
trackless desert desti- 
tute of all life save 
such lowly forms as 
the microscopic plant 
which produces the 
so-called " red snow." On the smooth plain of the interior no 
rock waste relieves the snow's dazzling whiteness ; no streams 
of running water are seen ; the silence is broken only by howl- 
ing storm winds and the rustle of the surface snow which they 
drive before them. Sounding with long poles, explorers find 




FIG. 90. Map of Greenland 
Glacier ice covers all but the areas shaded 



116 THE ELEMENTS OF GEOLOGY 

that below the powdery snow of the latest snowfall lie suc- 
cessive layers of earlier snows, which grow more and more 
compact downward, and at last have altered to impenetrable 
ice. The ice cap formed by the accumulated snows of uncounted 
centuries may well be more than a mile in depth. Ice thus 
formed by the compacting of snow is distinguished when in 
motion as glacier ice. 

The inland ice of Greenland moves. It flows with imper- 
ceptible slowness under its own weight, like a mass of some 
viscous or plastic substance, such as pitch or molasses candy, in 
all directions outward toward the sea. Near the edge it has so 

thinned that mountain peaks are 
laid bare, these islands in the sea 
FIG. 91. Hypothetic Cross Sec- of ice being known as nunata/cs. 

Down the valleys of the coastal 

belt it drains in separate streams of ice, or glaciers. The largest 
of these reach the sea at the head of inlets, and are therefore 
called tide glaciers. Their fronts stand so deep in sea water 
that there is visible seldom more than three hundred feet of the 
wall of ice, which in many glaciers must be two thousand and 
more feet high. From the sea walls of tide glaciers great frag- 
ments break off and float away as icebergs. Thus snows which 
fell in the interior of this northern land, perhaps many thou- 
sands of years ago, are carried in the form of icebergs to melt 
at last in the North Atlantic. 

Greenland, then, is being modeled over the vast extent of 
its ulterior not by streams of running water, as are regions in 
warm and humid climates, nor by currents of air, as are deserts 
to a large extent, but by a sheet of flowing ice. What the ice 
sheet is doing in the interior we may infer from a study of the 
separate glaciers into which it breaks at its edge. 

The smaller Greenland glaciers. Many of the smaller glaciers 
of Greenland do not reach the sea, but deploy on plains of sand 
and gravel. The edges of these ice tongues are often as abrupt 



THE WORK OF GLACIERS 



117 



as if sliced away with a knife (Fig. 92), and their structure is 
thus readily seen. They are stratified, their layers representing 
in part the successive snowfalls of the interior of the country. 
The upper layers are commonly white and free from stones; 
but the lower layers, to the height of a hundred feet or more, 
are dark with debris which is being slowly carried on. So 
thickly studded with stones is the base of the ice that it is 




FIG. 92. A Greenland Glacier 

sometimes difficult to distinguish it from the rock waste which 
has been slowly dragged beneath the glacier or left about its 
edges. The waste beneath and about the glacier is unsorted. 
The stones are of many kinds, and numbers of them have been 
ground to flat faces. Where the front of the ice has retreated 
the rock surface is seen to be planed and scored in places by 
the stones frozen fast in the sole of the glacier. 

We have now found in glacier ice an agent able to produce 
the drift of North America. The ice sheet of Greenland is now 



118 THE ELEMENTS OF GEOLOGY 

doing what we have seen was done in the recent past in our 
own land. It is carrying for long distances rocks of many 
kinds gathered, we may infer, over a large extent of country. 
It is laying down its load without assortment in unstratified 
deposits. It grinds down and scores the rock over which it 
moves, and in the process many of the pebbles of its load are 
themselves also ground smooth and scratched. Since this work 
can be done by no other agent, we must conclude that the 
northeastern part of our own continent was covered in the 
recent past by glacier ice, as Greenland is to-day. 

VALLEY GLACIERS 

The work of glacier ice can be most conveniently studied in 
the separate ice streams which creep down mountain valleys in 
many regions such as Alaska, the western mountains of the 
United States- and Canada, the Himalayas, and the Alps. As 
the glaciers of the Alps have been stvidied longer and more 
thoroughly than any others, we shall describe them in some 
detail as examples of valley glaciers in all parts of the world. 

Conditions of glacier formation. The condition of the great 
accumulation of snow to which glaciers are due that more 
or less of each winter's snow should be left over unmelted and 
unevaporated to the next is fully met in the Alps. There is 
abundant moisture brought by the winds from neighboring seas. 
The currents of moist air driven up the mountain slopes are 
cooled by their own expansion as they rise, and the moisture 
which they contain is condensed at a temperature at or below 
32 F., and therefore is precipitated in the form of snow. The 
summers are cool and their heat does not suffice to completely 
melt the heavy snow of the preceding winter. On the Alps 
the snow line the lower limit of permanent snow is 
drawn at about eight thousand five hundred feet above sea 
level. Above the snow line on the slopes and crests, where 




119 



120 



THE ELEMENTS OF GEOLOGY 



these are not too steep, the snow lies the year round and gathers 
in valley heads to a depth of hundreds of feet. 

This is but a small fraction of the thickness to which snow 
would be piled on the Alps were it not constantly being 
drained away. Below the snow fields which mantle the heights 
the mountain valleys are occupied by glaciers which extend as 
much as a vertical mile below the snow line. The presence 

in the midst of forests and 
meadows and cultivated 
fields of these tongues of 
ice, ever melting and yet 
from year to year losing 
none of their bulk, proves 
that their loss is made good 
in the only possible way. 
They are fed by snow fields 
above, whose surplus of 
snow they drain away in 
the form of ice. The pres- 
ence of glaciers below the 
snow line is a clear proof 
that, rigid and motionless 
as they appear, glaciers 
really are in constant motion 
down valley. 

The neve field. The head of an Alpine valley occupied by 
a glacier is commonly a broad amphitheater deeply filled with 
snow (Fig. 93). Great peaks tower above it, and snowy slopes 
rise on either side on the flanks of mountain spurs. From these 
heights fierce winds drift the snows into the amphitheater, and 
avalanches pour in their torrents of snow and waste. The snow 
of the amphitheater is like that of drifts in late winter after many 
successive thaws and freezings. It is made of hard grains and 
pellets and is called nve. Beneath the surface of the neve 




FIG. 94. Bergschrund of a Glacier 
in Colorado 



THE WORK OF GLACIERS 



121 



field and at its outlet the granular neve has been compacted 
to a mass of porous crystalline ice. Snow has been changed 
to neve, and neve to glacial ice, both by pressure, which drives 
the air from the interspaces of the snowflakes, and also by 
successive meltings and freezings, much as a snowball is packed 
in the warm hand and becomes frozen to a ball of ice. 

The bergschrund. The neve is in slow motion. It breaks 
itself loose from the thinner snows about it, too shallow to share 




FIG. 95. Sea Wall of the Muir Glacier, Alaska 

its motion, and from the rock rim which surrounds it, forming a 
deep fissure called the bergschrund, sometimes a score and more 
feet wide (Fig. 94). 

Size of glaciers. The ice streams of the Alps vary in size 
according to the amount of precipitation and the area of the 
neve fields which they drain. The largest of Alpine glaciers, 
the Aletsch, is nearly ten miles long and has an average width 
of about a mile. The thickness of some of the glaciers of the 
Alps is as much as a thousand feet. Giant glaciers more than 
twice the length of the longest in the Alps occur on the south 
slope of the Himalaya Mountains, which receive frequent 



122 



THE ELEMENTS OF GEOLOGY 



precipitations of snow from moist winds from the Indian Ocean. 
The best known of the many immense glaciers of Alaska, the 
Muir, has an area of about eight hundred square miles (Fig. 95). 



1 


234 


567 




" 


^o-o 


-o-'^ 


\ 




</'/ 
_-*-' / 




^ c --c 


.-o^'' 




1 





bed 



FIG. 90. Diagram showing Movement 
of Row of Stakes a, set in a 
direct line across the surface of a 
glacier ; b, c, and d, successive 
later positions of the stakes 



FIG. 97. Diagram showing Movement 
of Vertical Row of Stakes a, set 
on side of glacier 



Glacier motion. The motion of the glaciers of the Alps seldom 
exceeds one or two feet a day. Large glaciers, because of the 
enormous pressure of their weight and because of less marginal 
resistance, move faster than small ones. The Muir advances at 

the rate of seven 
feet a day, and some 
of the larger tide 
glaciers of Green- 
land are reported 
to move at the ex- 
ceptional rate of 
fifty feet and more 
in the same time. 
Glaciers move faster 
by day than by 
night, and in sum- 
mer than in winter. 
Other laws of glacier motion may be discovered by a study of 
Figures 96 and 97. It is important to remember that glaciers 
do not slide bodily over their beds, but urged by gravity move 
slowly down valley in somewhat the same way as would a 




FIG. 98. Crevasses of a Glacier, Canada 



THE WORK OF GLACIERS 



123 



stream of thick mud. Although small pieces of ice are brittle, 
the large mass of granular ice which composes a glacier acts 
as a viscous substance. 



\ 




FIG. 99. Longitudinal Section of a Portion of a 
Glacier, showing Transverse Crevasses 



FIG. 100. Map View of 
Marginal Crevasses 



Crevasses. Slight changes of slope in the glacier bed, and the differ- 
ent rates of motion in different parts, produce tensions under which the 
ice cracks and opens in great fissures called crevasses. At an abrupt 




FIG. 101. The Rhone Glacier, showing Radial Crevasses, the Alps 



124 



THE ELEMENTS OF GEOLOGY 



descent in the bed the ice is shattered into great fragments, which 
unite again below the icefall. Crevasses are opened on lines at right 
angles to the direction of the tension. Transverse crevasses are due to a 
convexity in the bed which stretches the ice lengthwise (Fig. 99). Mar- 
ginal crevasses are directed upstream and inwards ; radial crevasses are 
found where the ice stream deploys from some narrow valley and 
spreads upon some more open space. What is the direction of the 
tension which causes each and to what is it due? (Figs. 100 and 101.) 

Lateral and medial moraines. The surface of a glacier is 
striped lengthwise by long dark bands of rock debris. Those in 
the center are called the medial mo- 
raines. The one on either margin is a 
lateral moraine, and is clearly formed of 
waste which has fallen on the edge of 
the ice from the valley slopes. A medial 
moraine cannot be formed in this way, 
since no rock fragments can fall so far 

out from the sides. But following it 
FIG. 102. Map View of 

the Junction of Two up the glacial stream, one finds that a 

Branches of a Glacier medial moraine takes its beginning at 
are repre- the junction of the glacier and some 
tributary and is formed by the union 
of their two adjacent lateral moraines (Fig. 102). Each branch 
thus adds a medial moraine, and by counting the number of 
medial moraines of a trunk stream 

~ f ~fiit in . 

one may learn of how many [xT^C^T 71 ??^ 
branches it is composed. 




The moraines 
sented by broken lines 



Surface moraines appear in the FlG 103 Crogs Section of a 
lower course of the glacier as Glacier showing Lateral Mo- 
ridges, which may reach the ex- raines * * and Medial Mo- 

, . i i 1 , raines m. m 
ceptional height or one hundred 

feet. The bulk of such a ridge is ice. It has been protected 
from the sun by the veneer of moraine stuff ; while the glacier 
surface on either side has melted down at least the distance of 



THE WORK OF GLACIERS 125 

the height of the ridge. In summer the lowering of the glacial 
surface by melting goes on rapidly. In Swiss glaciers it has been 
estimated that the average lowering of the surface by melting 
and evaporation amounts to ten feet a year. As a moraine ridge 
grows higher and more steep by the lowering of the surface of 
the surrounding ice, the stones of its cover tend to slip down 




FIG. 104. Glacier with Medial Moraines, the Alps 
Is the ice moving from or towards the observer? 

its sides. Thus moraines broaden, until near the terminus of a 
glacier they may coalesce in a wide field of stony waste. 

Englacial drift. This name is applied to whatever debris is 
carried within the glacier. It consists of rock waste fallen on 
the neve and there buried by accumulations of snow, and of 
that engulfed in the glacier where crevasses have opened beneath 
a surface moraine. As the surface of the glacier is lowered by 
melting, more or less englacial drift is brought again to open 
air, and near the terminus it may help to bury the ice from 
view beneath a sheet of debris. 



126 THE ELEMENTS OF GEOLOGY 

The ground moraine. The drift dragged along at the gla- 
cier's base and lodged beneath it is known as the ground mo- 
raine. Part of the material of it has fallen down deep crevasses 
and part has been torn and worn from the glacier's bed and 
banks. While the stones of the surface moraines remain as 
angular as when they lodged on the ice, many of those of the 
ground moraine have been blunted on the edges and faceted 
and scratched by being ground against one another and the 
rocky bed. 

In glaciers such as those of Greenland, whose basal layers are well 
loaded with drift and whose surface layers are nearly clean, different 
layers have different rates of motion, according to the amount of drift 
with which they are clogged. One layer glides over another, and the 
stones inset in each are ground and smoothed and scratched. Usually 
the sides of glaciated pebbles are more worn than the ends, and the 
scratches upon them run with the longer axis of the stone. Why ? 

The terminal moraine. As a glacier is in constant motion, it 
brings to its end all of its load except such parts of the ground 
moraine as may find permanent lodgment beneath the ice. 
Where the glacier front remains for some time at one place, 
there is formed an accumulation of drift known as the terminal 
moraine. In valley glaciers it is shaped by the ice front to a 
crescent whose convex side is downstream. Some of the peb- 
bles of the terminal moraine are angular, and some are faceted and 
scored, the latter having come by the hard road of the ground 
moraine. The material of the dump is for the most part 
unsorted, though the water of the melting ice may find oppor- 
tunity to leave patches of stratified sands and gravels in the 
midst of the unstratified mass of drift, and the finer material 
is in places washed away. 

Glacier drainage. The terminal moraine is commonly breached 
by a considerable stream, which issues from beneath the ice 
by a tunnel whose portal has been enlarged to a beautiful 




127 



128 



THE ELEMENTS OF GEOLOGY 



archway by melting in the sun and the warm air (Fig. 107). The 
stream is gray with silt and loaded with sand and gravel washed 
from the ground moraine. " Glacier milk " the Swiss call this 
muddy water, the gray color of whose silt proves it rock flour 
freshly ground by the ice from the unoxidized sound rock of its 




FIG. 100. Heavy Moraine about the Terminus of a Glacier in the 
Rocky Mountains of Canada 

Account for the fact that the morainic ridge rises considerably ahove the 
'surface of the ice 

bed, the mud of streams being yellowish when it is washed 
from the oxidized mantle of waste. Since glacial streams are 
well loaded with waste due to vigorous ice erosion, the valley 
in front of the glacier is commonly aggraded to a broad, flat 
floor. These outwash deposits are known as valley drift. 

The sand brought out by streams from beneath a glacier differs from 
river sand in that it consists of freshly broken angular grains. Why? 

The stream derives its water chiefly from the surface melting of 
the glacier. As the ice is touched by the rays of the morning sun in 



THE WORK OF GLACIERS 



129 




summer, water gathers in pools, and rills trickle and unite in brooklets 
which melt and cut shallow channels in the blue ice. The course of 
these streams is short. Soon they plunge into deep wells cut by their 
whirling waters where some crevasse has begun to open across their 
path. These wells lead into chambers and tunnels by which sooner 
or later their waters find way 
to the rock floor of the valley 
and there unite in a subglacial 
stream. 

The lower limit of gla- 
ciers. The glaciers of a region 
do not by any means end 
at a uniform height above 
sea level. Each terminates 
where its supply is balanced 
by melting. Those therefore 
which are fed by the largest 
and deepest neves and those 
also w r hich are best protected 
from the sun by a northward exposure or by the depth of their 
inclosing valleys flow to lo\ver levels than those whose supply 
is less and whose exposure to the sun is greater. 

A series of cold, moist years, with an abundant snowfall, 
causes glaciers to thicken and advance ; a series of warm, dry 
years causes them to wither and melt back. The variation 
in glaciers is now carefully observed in many parts of the 
world. The Muir glacier has retreated two miles in twenty 
years. The glaciers of the Swiss Alps are now for the most part 
melting back, although a well-known glacier of the eastern 
Alps, the Yernagt, advanced five hundred feet in the year 
1900, and was then plowing up its terminal moraine. 

How soon would you expect a glacier to advance after its ne've' fields 
have been swollen with unusually heavy snows, as compared with the 
time needed for the flood of a large river to reach its mouth after 
heavy rains upon its headwaters ? 



FIG. 107. Subglacial Stream issuing 
from Tunnel in the Ice, Norway 



130 



THE ELEMENTS OF GEOLOGY 



On the surface of glaciers in summer time one may often see large 
stones supported by pillars of ice several feet in height (Fig. 108). 
These " glacier tables " commonly slope more or less strongly to the 
south, and thus may be used to indicate 
roughly the points of the compass. Can you 
explain their formation and the direction 
of their slope ? On the other hand, a small 
and thin stone, or a patch of dust, lying 
on the ice, tends to sink a few inches into it. 
FIG. 108. A Glacier Table why? 

In what respects is a valley glacier like a mountain stream which 
flows out upon desert plains? 

Two confluent glaciers do not mingle their currents as do two con- 
fluent rivers. What characteristics of surface moraines prove this fact? 

What effect would you expect the laws of glacier motion to have 
on the slant of the sides of transverse crevasses ? 





FIG. 109. Map of Malaspina Glacier, Alaska 

A trunk glacier has four medial moraines. Of how many tributaries 
is it composed? Illustrate by diagram. 

State all the evidences which you have found that glaciers move. 

If a glacier melts back with occasional pauses up a valley, what 
records are left of its retreat ? 



THE WORK OF GLACIERS 



131 



PIEDMONT GLACIEKS 

The Malaspina glacier. Piedmont (foot of the mountain) 
glaciers are, as the name implies, ice fields formed at the foot of 
mountains by the confluence of valley glaciers. The Malaspina 
glacier of Alaska, the typical glacier of this kind, is seventy 
miles wide and stretches for thirty miles from the foot of the 
Mount Saint Elias range to the shore of the Pacific Ocean. The 
valley glaciers which unite and spread to form this lake of ice lie 
above the snow line and their moraines are concealed beneath 
neve. The central 
area of the Malas- 
pina is also free 
from debris ; but 
on the outer edge 
large quantities of 
englacial drift are 




FIG. 110. Outwash Plain, the Delta of the 
Yahtse River, Alaska 



exposed by surface 
melting and form a 
belt of morainic 
waste a few feet 
thick and several 
miles wide, covered 
in part with a lux- 
uriant forest, be- 
neath which the ice is in places one thousand feet in depth. 
The glacier here is practically stagnant, and lakes a few hundred 
yards across, which could not exist were the ice in motion and 
broken with crevasses, gather on their beds sorted waste from 
the moraine. The streams which dram the glacier have cut 
their courses in englacial and subglacial tunnels ; none flow for 
any distance on the surface. The largest, the Yahtse River, 
issues from a high archway in the ice, a muddy torrent one 
hundred feet wide and twenty feet deep, loaded with sand and 



132 THE ELEMENTS OF GEOLOGY 

stones which it deposits in a broad outwash plain (Fig. 110). 
Where the ice has retreated from the sea there is left a hum- 
mocky drift sheet with hollows filled with lakelets. These 
deposits help to explain similar hummocky regions of drift 
and similar plains of coarse, water-laid material often found in 
the drift-covered area of the northeastern United States. 

THE GEOLOGICAL WORK OF GLACIER ICE 

The sluggish glacier must do its work in a different way 
from the agile river. The mountain stream is swift and small, 
and its channel occupies but a small portion of the valley. 
The glacier is slow and big; its rate of motion may be 
less than a millionth of that of running water over the same 
declivity, and its bulk is proportionately large and fills the 
valley to great depth. Moreover, glacier ice is a solid body 
plastic under slowly applied stresses, while the water of rivers 
is a nimble fluid. 

Transportation. Valley glaciers differ from rivers as carriers 
in that they float the major part of their load upon their surface, 
transporting the heaviest bowlder as easily as a grain of sand ; 
while streams push and roll much of their load along their beds, 
and their power of transporting waste depends solely upon their 
velocity. The amount of the surface load of glaciers is limited 
only by the amount of waste received from the mountain slopes 
above them. The moving floor of ice stretched high across a 
valley sweeps along as lateral moraines much of the waste 
which a mountain stream would let accumulate in talus and 
alluvial cones. 

While a valley glacier carries much of its load on top, an ice 
sheet, such as that of Greenland, is free from surface debris, 
except where moraines trail away from some nunatak. If at its 
edge it breaks into separate glaciers which drain down mountain 
valleys, these tongues of ice will carry the selvages of waste 



THE WORK OF GLACIERS 133 

common to valley glaciers. Both ice sheets and valley glaciers 
drag on large quantities of rock waste in their ground moraines. 

Stones transported by glaciers are sometimes called erratics. 
Such are the bowlders of the drift of our northern states. 
Erratics may be set down in an insecure position on the melting 
of the ice. 

Deposit. Little need be added here to what has already been 
said of ground and terminal moraines. All strictly glacial 
deposits are unstratified. The load laid down at the end of a 
glacier in the terminal moraine is loose in texture, while the 
drift lodged beneath the glacier as ground moraine is often an 
extremely dense, stony clay, having been compacted under the 
pressure of the overriding ice. 

Erosion. A glacier erodes its bed and banks in two ways, 
by abrasion and by plucking. 

The rock bed over which a glacier has moved is seen in places 
to have been abraded, or ground away, to smooth surfaces which 
are marked by long, straight, parallel scorings aligned with the 
line of movement of the ice and varying in size from hair lines and 
coarse scratches to exceptional furrows several feet deep. Clearly 
this work has been accomplished by means of the sharp sand, 
the pebbles, and the larger stones with which the base of the 
glacier is inset, and which it holds in a firm grasp as running 
water cannot. Hard and fine-grained rocks, such as granite and 
quartzite, are often not only ground down to a smooth surface 
but are also highly polished by means of fine rock flour worn 
from the glacier bed. 

In other places the bed of the glacier is rough and torn. The 
rocks have been disrupted and their fragments have been carried 
away, a process known as plucking. Moving under immense 
pressure the ice shatters the rock, breaks off projections, presses 
into crevices and wedges the rocks apart, dislodges the blocks 
into which the rock is divided by joints and bedding planes, and 
freezing fast to the fragments drags them on. In this work the 



134 



THE ELEMENTS OF GEOLOGY 



freezing and thawing of subglacial waters in any cracks and 
crevices of the rock no doubt play an important part. Pluck- 
ing occurs especially where the bed rock is weak because of 
close jointing. The product of plucking is bowlders, while the 
product of abrasion is fine rock flour and sand. 

Is the ground moraine of Figure 87 due chiefly to abrasion or to 
plucking ? 

Roches moutonnees and rounded hills. The prominences left 
between the hollows due to plucking are commonly ground 

down and rounded on 
the s.toss side, the 
side from which the ice 
advances, and some- 
times on the opposite, 
the lee side, as well. In 
this way the bed rock 
often comes to have a 
billowy surface known 
as roches moutonnees 
(sheep rocks). Hills 
overridden by an ice 
sheet often have simi- 
larly rounded contours on the stoss side, while on the lee side 
they may be craggy, either because of plucking or because here 
they have been less worn from their initial profile (Fig. 112). 

The direction of glacier movement. The direction of the flow 
of vanished glaciers and ice sheets is recorded both in the dif- 
ferences just mentioned in the profiles of overridden hills and 
also in the minute details of the glacier trail. 

Flint nodules or other small prominences in the bed rock are 
found more worn on the stoss than on the lee side, where indeed 
they may have a low cone of rock protected by them from 
abrasion. Cavities, on the other hand, have their edges worn on 
the lee side and left sharp upon the stoss. 




FIG. 111. Roches Moutonne'es, Bronx Park, 
New York 



THE WORK OF GLACIERS 



135 




Surfaces worn and torn in the ways which we have mentioned 
are said to be glaciated. But it must not be supposed that a 
glacier everywhere glaciates its bed. Although in places it acts 
as a rasp or as a pick, in others, and especially where its pressure 
is least, as near the terminus, it moves over its bed in the manner 
of a sled. Instances are known where glaciers have advanced 
over deposits of sand and gravel without disturbing them to 
any notable degree. Like a river, a glacier does not everywhere 
erode. In places it 
leaves its bed un- 
disturbed and in 
places aggrades it 
by deposits of the 
ground moraine. 

Cirques. Valley 
glaciers commonly 
head, as we have 
seen, in broad am- 
phitheaters deeply filled with snow and ice. On mountains now 
destitute of glaciers, but whose glaciation shows that they have 
supported glaciers in the past, there are found similar crescentic 
hollows with high, precipitous walls and glaciated floors. Their 
floors are often basined and hold lakelets whose deep and quiet 
waters reflect the sheltering ramparts of rugged rock which 
tower far above them. Such mountain hollows are termed 
cirques. As a powerful spring wears back a recess in the val- 
ley side where it discharges, so the fountain head of a glacier 
gradually wears back a cirque. In its slow movement the neve 
field broadly scours its bed to a flat or basined floor. Mean- 
while the sides of the valley head are steepened and driven back 
to precipitous walls. For in winter the crevasse of the berg- 
schrund which surrounds the neve field is filled with snow and 
the neve is frozen fast to the rocky sides of the valley. In early 
summer the neve tears itself free, dislodging and removing 



FIG. 112. A Glaciated Hill, Norway. Sharp 
Cirque-Cut Mountain Peaks in the Distance 




136 



THE WORK OF GLACIERS 



137 



any loosened blocks, and the open fissure of the bergschrund 
allows frost and other agencies of weathering to attack the un- 
protected rock. As cirques are thus formed and enlarged the 
peaks beneath which they lie are sharpened^ and the mountain 
crests are scalloped and cut back from either side to knife-edged 
ridges (Figs. 113 and 93). 

In the western mountains of the United States many cirques, 
now empty of nev6 and glacier ice, and known locally as 
" basins," testify to the fact that in recent times the snow line 
stood beneath the levels of their floors, and thus far below its 
present altitude. 

Glacier troughs. The channel worn to accommodate the big 
and clumsy glacier differs markedly from the river valley cut 




FIG. 114. A Glacier Trough, Montana 

as with a saw by the narrow and flexible stream and widened 
by the weather and the wash of rains. The valley glacier may 
easily be from one thousand to three thousand feet deep and 
from one to three miles wide. Such a ponderous bulk of slowly 



138 THE ELEMENTS OF GEOLOGY 

moving ice does not readily adapt itself to sharp turns and a 
narrow bed. By scouring and plucking all resisting edges it de- 
velops a fitting channel with a wide, flat floor, and steep, smooth 
sides, above which are seen the weathered slopes of stream-worn 
mountain valleys. Since the trunk glacier requires a deeper 
channel than do its branches, the bed of a branch glacier enters 
the main trough at some distance above the floor of the latter, 




FIG. 115. Lynn Canal, Alaska, a Fjord 

although the surface of the two ice streams may be accordant. 
Glacier troughs can be studied best where large glaciers have 
recently melted completely away, as is the case in many valleys 
of the mountains of the western United States and of central 
and northern Europe (Fig. 114). The typical glacier trough, as 
shown in such examples, is U-shaped, with a broad, flat floor, 
and high, steep walls. Its walls are little broken by projecting 
spurs and lateral ravines. It is as if a V-valley cut by a river 
had afterwards been gouged deeper with a gigantic chisel, wid- 
ening the floor to the width of the chisel blade, cutting back the 
spurs, and smoothing and steepening the sides. A river valley 



THE WORK OF GLACIERS 



139 



could only be as wide-floored as this after it had long been 
worn down to grade. 

The floor of a glacier trough may not be graded ; it is often 
interrupted by irregular steps perhaps hundreds and even a 
thousand feet in height, 
over which the stream that 
now drains the valley tum- 
bles in waterfalls. Eeaches 
between the steps are often 
basined. Lakelets may 
occupy hollows excavated 
in solid rock, and other lakes 
may be held behind terminal 
moraines left as dams across 
the valley at pauses in the 
retreat of the glacier. 

Fjords are glacier troughs 
now occupied in part or wholly 
by the sea, either because they 
were excavated by a tide glacier 
to their present depth below 
sea level, or because of a sub- 
mergence of the land. Their 
characteristic form is that of a 
long, deep, narrow bay with 




FIG. 116. A, V-River Valley, with Valley 
of Tributary joining it at Accordant 
Level ; B, the Same changed after 
Long Glaciation to a Glacier Trough 
with Hanging Valley 



steep rock walls and basined 

floor (Fig. 115). Fjords are 

found only in regions which have suffered glaciation, such as Norway 

and Alaska. 

Hanging valleys. These are lateral valleys which open on 
their main valley some distance above its floor. They are con- 
spicuous features of glacier troughs from which the ice has van- 
ished; for the trunk glacier in widening and deepening its 
channel cut its bed below the bottoms of the lateral valleys 
(Fig. 116). 



140 



THE ELEMENTS OF GEOLOGY 



Since the mouths of hanging valleys are suspended on the 
walls of the glacier trough, their streams are compelled to 
plunge down its steep, high sides in waterfalls. Some of the 
loftiest and most beautiful waterfalls of the world leap from 
hanging valleys, among them the celebrated Staubbach of the 
Lauterbrunnen valley of Switzerland, and those of the fjords 
of Norway and Alaska (Fig. 117). 

Hanging valleys are found also in river gorges where the 
smaller tributaries have not been able to keep pace with a 

strong master stream in 
cutting down their beds. 
In this case, however, 
they are a mark of ex- 
treme youth ; for, as the 
trunk stream approaches 
grade and its velocity 
and power to erode its 
bed decrease, the side 
streams soon cut back 
their falls and wear 
their beds at their 
mouths to a common 
level with that of the main river. The Grand Canyon of the 
Colorado must be reckoned a young valley. At its base it nar- 
rows to scarcely more than the width of the river, and yet its 
tributaries, except the very smallest, enter it at a common level. 

Why could not a wide-floored valley, such as a glacier trough, with 
hanging valleys opening upon it, be produced in the normal develop- 
ment of a river valley ? 

The troughs of young and of mature glaciers. The features of a glacier 
trough depend much on the length of time the preexisting valley was 
occupied with ice. During the infancy of a glacier, we may believe, the 
spurs of the valley which it fills are but little blunted and its bed is 
but little broken by steps. In youth the glacier develops icefalls, as a 




FIG. 117. Hanging Valley on the Wall of 
a Fjord, Norway 



THE WORK OF GLACIERS 141 

river in youth develops waterfalls, and its bed becomes terraced with great 
stairs. The mature glacier, like the mature river, has effaced its falls and 
smoothed its bed to grade. It has also worn back the projecting spurs 
of its valley, making itself a wide channel with smooth sides. The bed 
of a mature glacier may form a long basin, since it abrades most in its 
upper and middle course, where its weight and motion are the greatest. 
Near the terminus, where weight and motion are the least, it erodes 
least, and may instead deposit a sheet of ground moraine, much as a 
river builds a flood plain in the same part of its course as it approaches 
maturity. The bed of a mature glacier thus tends to take the form of a 
long, relatively narrow basin, across whose lower end may be stretched 
the dam of the terminal moraine. On the disappearance of the ice the 
basin is filled with a long, narrow lake, such as Lake Chelan in Wash- 
ington and many of the lakes in the Highlands of Scotland. 

Piedmont glaciers apparently erode but little. Beneath their lake- 
like expanse of sluggish or stagnant ice a broad sheet of ground 
moraine is probably being deposited. 

Cirques and glaciated valleys rapidly lose their characteris- 
tic forms after the ice has withdrawn. The weather destroys 
all smoothed, polished, and scored surfaces which are not pro- 
tected beneath glacial deposits. The oversteepened sides of the 
trough are graded by landslips, by talus slopes, and by alluvial 
cones. Morainic heaps of drift are dissected and carried away. 
Hanging valleys and the irregular bed of the trough are both 
worn down to grade by the streams which now occupy them. 
The length of time since the retreat of the ice from a mountain 
valley may thus be estimated by the degree to which the destruc- 
tion of the characteristic features of the glacier trough has been 
carried. 

In Figure 104 what characteristics of a glacier trough do you notice? 
What inference do you draw as to the former thickness of the glacier ? 

Name all the evidences you would expect to find to prove the fact 
that in the recent geological past the valleys of the Alps contained far 
larger glaciers than at present, and that on the north of the Alps the 
ice streams united in a piedmont glacier which extended across the 
plains of Switzerland to the sides of the Jura Mountains. 



142 



THE ELEMENTS OF GEOLOGY 



The relative importance of glaciers and of rivers. Powerful 
as glaciers are, and marked as are the land forms which they 
produce, it is easy to exaggerate their geological importance as 
compared with rivers. Under present climatic conditions they 
are confined to lofty mountains or polar lands. Polar ice 
sheets are permanent only so long as the lands remain on 
which they rest. Mountain glaciers can stay only the brief 
tune during which the ranges continue young and high. As 

lofty mountains, 
such as the Sel- 
kirks and the 
Alps, are lowered 
by frost and 
glacier ice, the 
snowfall will de- 




FIG. 118. Longitudinal Section of a Tide Glacier occu- 
pying a Fjord and discharging Icebergs 



Dotted line, sea level 



crease, the line 
of permanent 
snow will rise, 
and as the mountain hollows in which snow may gather are 
worn beneath the snow line, the glaciers must disappear. Under 
present climatic conditions the work of glaciers is therefore both 
local and of short duration. 

Even the glacial epoch, during which vast ice sheets depos- 
ited drift over northeastern North America, must have been 
brief as well as recent, for many lofty mountains, such as the 
Ptockies and the Alps, still bear the marks of great glaciers 
which then filled their valleys. Had the glacial epoch been 
long, as the earth counts time, these mountains would have 
been worn low by ice ; had the epoch been remote, the marks 
of glaciation would already have been largely destroyed by 
other agencies. 

On the other hand, rivers are well-nigh universally at work 
over the land surfaces of the globe, and ever since the dry land 
appeared they have been constantly engaged in leveling the 



THE WORK OF GLACIERS 143 

continents and in delivering to the seas the waste which there 
is huilt into the stratified rocks. 

Icebergs. Tide glaciers, such as those of Greenland and Alaska, 
are able to excavate their beds to a considerable distance below 
sea level. From their fronts the buoyancy of sea water raises 
and breaks away great masses of ice which float out to sea as 
icebergs. Only about one seventh of a mass of glacier ice floats 
above the surface, and a berg three hundred feet high may be 
estimated to have been detached from a glacier not less than 
two thousand feet thick where it met the sea. 

Icebergs transport on their long journeys whatever drift they 
may have carried when part of the glacier, and scatter it, as 
they melt, over the ocean floor. In this way pebbles torn by the 
inland ice from the rocks of the interior of Greenland and gla- 
ciated during their carriage in the ground moraine are dropped 
at last among the oozes of the bottom of the North Atlantic. 



CHAPTEE VI 



THE WORK OF THE WIND 

We are now to study the geological work of the currents of 
the atmosphere, and to learn how they erode, and transport and 
deposit waste as they sweep over the land. Illustrations of the 
wind's work are at hand in dry weather on any windy day. 

Clouds of dust are raised 
from the street and 
driven along by the gale. 
Here the roadway is 
swept bare; and there, 
in sheltered places, the 
dust settles in little 
windrows. The erosive 
power of waste-laden cur- 
rents of air is suggested 
as the sharp grains of 
flying sand sting one's 
face or clatter against 
the window. In the 
country one sometimes 




FIG. 119. A Sandy Region in a Desert, 
the Sahara 



Account for the mounds of sand on which the 
clumps of brush are growing 



sees the dust whirled in 
clouds from dry, plowed 
fields in spring and left in the lee of fences in small drifts 
resembling in form those of snow in winter. 

The essential conditions for the wind's conspicuous work are 
illustrated in these simple examples; they are aridity and the 
absence of vegetation. In humid climates these conditions are 
only rarely and locally met ; for the most part a thick growth 

144 



THE WORK OF THE WIND 145 

of vegetation protects the moist soil from the wind with a cover 
of leaves and stems and a mattress of interlacing roots. But 
in arid regions either vegetation is wholly lacking, or scant 
growths are found huddled in detached clumps, leaving inter- 
spaces of unprotected ground (Fig. 119). Here, too, the mantle 
of waste, which is formed chiefly under the action of temperature 
changes, remains dry and loose for long periods. Little or no 
moisture is present to cause its particles to cohere, and they 
are therefore readily lifted and drifted by the wind. 

TRANSPORTATION BY THE WIND 

In the desert the finer waste is continually swept to and fro 
by the ever-shifting wind. Even in quiet weather the air heated 
by contact with the hot sands rises in whirls, and the dust is 
lifted in stately columns, sometimes as much as one thousand 
feet in height, which march slowly across the plain. In storms 
the sand is driven along the ground in a continuous sheet, 
while the air is filled with dust. Explorers tell of sand storms 
in the deserts of central Asia and Africa, in which the air grows 
murky and suffocating. Even at midday it may become dark 
as night, and nothing can be heard except the roar of the 
blast and the whir of myriads of grains of sand as they fly 
past the ear. 

Sand storms are by no means uncommon in the arid regions of 
the western United States. In a recent year, six were reported from 
Yuma, Arizona. Trains on transcontinental railways are occasionally 
blockaded by drifting sand, and the dust sifts into closed passenger 
coaches, covering the seats and floors. After such a storm thirteen car 
loads of sand were removed from the platform of a station on a western 
railway. 

Dust falls. Dust launched by upward-whirling winds on the 
swift currents of the upper air is often blown for hundreds of 
miles beyond the arid region from which it was taken. Dust 
falls from western storms are not unknown even as far east as 



146 



THE ELEMENTS OF GEOLOGY 



the Great Lakes. In 1896 a "black snow" fell in Chicago, 
and in another dust storm in the same decade the amount of 
dust carried in the air over Rock Island, 111., was estimated at 
more than one thousand tons to the cubic mile. 




FIG. 120. A Tract of Kocky Desert, Arabia 

By what process have these rocks been broken up ? Why is finer 
waste here absent ? 

In March, 1901, a cyclonic storm carried vast quantities of dust from 
the Sahara northward across the Mediterranean to fall over southern 
and central Europe. On March 8th dust storms raged in southern 
Algeria; two days later the dust fell in Italy; and on the llth it 
had reached central Germany and Denmark. It is estimated that in 
these few days one million eight hundred thousand tons of waste were 
carried from northern Africa and deposited on European soil. 

We may see from these examples the importance of the wind 
as an agent of transportation, and how vast in the aggregate 
are the loads which it carries. There are striking differences 
between air and water as carriers of. waste. Eivers flow in fixed 



THE WORK OF THE WIND 



147 



and narrow channels to definite goals. The channelless streams 
of the air sweep across broad areas, and, shifting about continu- 
ally, carry their loads back and forth, now in one direction and 
now in another. 

WIND DEPOSITS 

The mantle of waste of deserts is rapidly sorted by the wind. 
The coarser rubbish, too heavy to be lifted into the air, is left 
to strew wide tracts with residual gravels (Fig. 120). The sand 
derived from the disintegration of desert rocks gathers in vast 
fields. About one eighth of the surface of the Sahara is said 
to be thus covered with drifting sand. In desert mountains, as 
those of Sinai, it lies like fields of snow in the high valleys 
below the sharp peaks. On more level tracts it accumulates 
in seas of sand, sometimes, as in the deserts of Arabia, two 
hundred and more feet deep. 

Dunes. The sand thus accumulated by the wind is heaped 
in wavelike hills called dunes. In the desert of northwestern 
India, where the prev- 
alent wind is of great 
strength, the sand is 
laid in longitudinal 
dunes, i.e. in stripes 
running parallel with 
the direction of the 
wind; but commonly 
dunes lie, like ripple 
marks, transverse to the 
wind current. On the 
windward side they 







FIG. 121. Longitudinal Dunes, Desert of 
Northwestern India 



Scale, 1 inch = 3 miles 

show a long, gentle slope, up which grains of sand can readily 
be moved ; while to the lee their slope is frequently as great 
as the angle of repose (Fig. 122). Dunes whose sands are not 
fixed by vegetation travel slowly with the wind; for their 



148 



THE ELEMENTS OF GEOLOGY 




material is ever 
shifted forward as 
the grains are driven 
up the windward 
slope and, falling 
over the crest, are 
deposited in slanting 
layers in the quiet of 

"flip 1 PP 

FIG. 122. A Transverse Dune, Seven Mile Beach, L 

New Jersey Like river deposits, 

Account for the difference of slope in the two sides of wind-bio wn Sands 

the dune. Is the dune marching? In what direc- are stratified since 
tion? With what effect? Do the ridges of the 

ripple marks upon the dune extend along it or they are laid by CUT- 

athwart it? Why? rentg of ^ varying 

in intensity, and therefore in transporting power, which carry 
now finer and now coarser materials and lay them, down where 
their velocity is 
checked (Fig. 123). 
Since the wind varies 
in direction, the 
strata dip in vari- 
ous directions. They 
also dip at various 
angles, according to 
the inclination of the 
surface on which 
they were laid. 

Dunes occur not 
only in arid regions 
but also wherever 
loose sand lies un- 
protected by vegeta- 
tion from the wind. 
From the beaches of 




FIG. 123. Stratified Wind-Blown Sands, 
Bermuda Islands 

These islands are made wholly of limestone, the prod- 
uct of reef-building corals, and of plants which 
also secrete carbonate of lime from the sea water. 
The limestone sand of the beaches has been blown 
up into great dunes, some more than two hundred 
feet in height. Much of the loose dune sand has 
been changed to firm rock by percolating waters, 
which have dissolved some of the limestone and 
deposited it again as a cement between the grains 



THE WORK OF THE WIND 



149 



sea and lake shores the wind drives inland the surface sand 

left dry between tides and after storms, piling it in dunes which 

may invade forests and fields and 

bury villages beneath their slowly 

advancing waves. On flood plains FIG. 124. Cross Section of Trans- 

during summer droughts river de- ^ d Dune ater Revereal f 

posits are often worked over bjr JWmw djagram 8howing by 

the wind ; the Sand is heaped in dotted line the original outline 

hummocks and much of the fine of the dune 

silt is caught and held by the forests and grassy fields of the 

bordering hills. 

The sand of shore dunes differs little in composition and the 
shape of its grains from that of the beach from which it was 




FIG. 125. Dune Sands, Shore of Lake Michigan 

Account for the dead forest, for its leaning tree trunks. Is the lake shore 
to the right or left? What has been the history of the landscape? 

derived. But in deserts, by the long wear of grain on grain as 
they are blown hither and thither by the wind, all soft minerals 
are ground to powder and the sand comes to consist almost 
wholly of smooth round grains of hard quartz. 



150 



THE ELEMENTS OF GEOLOGY 



Some marine sandstones, such as the St. Peter sandstone of 
the upper Mississippi valley, are composed so entirely of polished 
spherules of quartz that it has been believed by some that their 
grains were long blown about in ancient deserts before they were 
deposited in the sea. 

Dust deposits. As desert sands are composed almost wholly 
of quartz, we may ask what has become of the softer minerals 
of which the rocks whose disintegration has supplied the sand 

were in part, and 

often in large part, 
composed. The 
softer minerals have 
been ground to 
powder, and little 
by little the quartz 
sand also is worn by 
attrition to fine dust. 
Yet dust deposits are 
scant and few in 
great deserts such 
as the Sahara. The 
finer waste is blown 




FIG. 126. Crescentic Sand Dunes, Valley of the 
Columbia River 



Did the wind which shaped them hlow from the left 

or from the right ? beyond its limits and 

laid in adjacent oceans, where it adds to the muds and oozes of 
their floors, and on bordering steppes and forest lands, where it 
is bound fast by vegetation and slowly accumulates in deposits 
of unstratified loose yellow earth. The fine waste of the Sahara 
has been identified in dredgings from the bottom of the Atlantic 
Ocean, taken hundreds of mile* from the coast of Africa. 

Loess. In northern China an area as large as France is deeply 
covered with a yellow pulverulent earth called loess (German, 
loose), which many consider a dust deposit blown from the great 
Mongolian desert lying to the west. Loess mantles the recently 
uplifted mountains to the height of eight thousand feet and 



THE WORK OF THE WIND 



151 



descends on the plains nearly to sea level. Its texture and lack 
of stratification give it a vertical cleavage ; hence it stands in 
steep cliffs on the sides of the deep and narrow trenches which 
have been cut in it by streams. 

On loess hillsides in China are thousands of villages whose cavelike 
dwellings have been excavated in this soft, yet firm, dry loam. While 
dust falls are common at the present time in this region, the loess is 
now being rapidly denuded by streams, and its yellow silt gives name 
to the muddy Hwang-ho (Yellow River), and to the Yellow Sea, whose 
waters it discolors for scores of miles from shore. 

Wind deposits both of dust and of sand may be expected to 
contain the remains of land shells, bits of wood, and bones of 
land animals, testifying to the fact that they were accumulated 
in open air and not hi the sea or hi bodies of fresh water. 

WIND EROSION 




FIG. 127. \Vind-Carved Rocks, Arizona 

Sand-laden currents of air abrade and smooth and polish 
exposed rock surfaces, acting in much the same way as does the 



152 



THE ELEMENTS OF GEOLOGY 



jet of steam fed with sharp sand, which is used in the manu- 
facture of ground glass. Indeed, in a single storm at Cape Cod 
a plate glass of a lighthouse was so ground by flying sand that 
its transparency was destroyed and its removal made necessary. 

Telegraph poles and wires whetted by wind-blown sands are 
destroyed within a few years. In rocks of unequal resistance the 

harder parts are left in relief, while the 
softer are etched away. Thus in the pass 
of San Bernardino, CaL, through which 
strong winds stream from the west, crys- 
tals of garnet are left projecting on deli- 
cate rock fingers from the softer rock in 
which they were imbedded. 

Wind-carved pebbles are characteristic- 
ally planed, the facets meeting along a 
summit ridge or at a point like that of 
a pyramid. We may suppose that these 
facets were ground by prevalent winds 
from certain directions, or that from time 
to time the stone was undermined and 
rolled over as the sand beneath it was 
blown away on the windward side, thus 
exposing fresh surfaces to the driving 
sand. Such wind-carved pebbles are sometimes found in ancient .rocks 
and may be accepted as evidence that the sands of which the rocks are 
composed were blown about by the wind. 

Deflation. In the denudation of an arid region, wind erosion 
is comparatively ineffective as compared with deflation (Latin, 
de, from ; flare, to blow), a term by which is meant the con- 
stant removal of waste by the wind, leaving the rocks bare to 
the continuous attack of the weather. In moist climates denu- 
dation is continually impeded by the mantle of waste and its 
cover of vegetation, and the land surface can be lowered no 
faster than the waste is removed by running water. Deep 
residual soils come to protect all regions of moderate slope, 
concealing from view the rock structure, and the various forms 




FIG. 128. A Wind-Carved 
Pebble, Cape Cod 



THE WORK OF THE WIND 



153 



of the land are due more to the agencies of erosion and trans- 
portation than to differences in the resistance of the under- 
lying rocks. 

But in arid regions the mantle is rapidly removed, even from 
well-nigh level plains and plateaus, by the sweep of the wind 
and the wash of occasional rains. The geological structure of 
these regions of naked rock can be read as far as the eye can see, 
and it is to this structure that the forms of the land are there 




FIG. 129. Mesa Verde, Colorado 

In the distance on the left are high volcanic mountains. On the ex- 
treme right are seen outliers of strata which once covered the region of 
the mesa 

largely due. In a land mass of horizontal strata, for example, 
any softer surface rocks wear down to some underlying, resist- 
ant stratum, and this for a while forms the surface of a level 
plateau (Fig. 129). The edges of the capping layer, together with 
those of any softer layers beneath it, wear back in steep cliffs, 
dissected by the valleys of wet-weather streams and often swept 
bare to the base by the wind. As they are little protected 
by talus, which commonly is removed about as fast as formed, 
these escarpments and the walls of the valleys retreat indefi- 
nitely, exposing some hard stratum beneath which forms the 
floor of a widening terrace. 

The high plateaus of northern Arizona and southern Utah 
(Fig. 130), north of the Grand Canyon of the Colorado Eiver, are 



154 THE ELEMENTS OF GEOLOGY 

composed of stratified rocks more than ten thousand feet thick 
and of very gentle inclination northward. From the broad plat- 
form in which the canyon has been cut rises a series of gigantic 
stairs, which are often more than one thousand feet high and 
a score or more of miles in breadth. The retreating escarp- 
ments, the cliff's of the mesas and buttes which they have left 
behind as outliers, and the walls of the ravines are carved 
into noble architectural forms into cathedrals, pyramids, 
amphitheaters, towers, arches, and colonnades by the processes 




EIG. 130. North-South Section, Eighty-Five Miles Long, across the 
Plateau North of the Grand Canyon of the Colorado River, Ari- 
zona, showing Retreating Escarpments 

O, outliers ; V, canyon of the Colorado ; A-H, rock systems from the 
Archaean to the Tertiary ; P, platform of the plateau from which the 
once overlying rocks have been stripped ; dotted lines indicate probable 
former extension of the strata. How thick is the mass of strata which 
has been removed from over the platform ? Has this work been accom- 
plished while the Colorado River has been cutting its present canyon? 

of weathering aided by deflation. It is thus by the help of the 
action of the wind that great plateaus in arid regions are dis- 
sected and at last are smoothed away to waterless plains, either 
composed of naked rock, or strewed with residual gravels, or 
covered with drifting residual sand. 

The specific gravity of air is i that of water. How does this fact 
affect the weight of the material which each can carry at the same 
velocity ? 

If the rainfall should lessen in your own state to from five to ten 
inches a year, what changes would take place in the vegetation of the 
country? in the soil? in the streams? in the erosion of valleys? in the 
agencies chiefly at work in denuding the land ? 

In what way can a wind-carved pebble be distinguished from a river- 
worn pebble ? from a glaciated pebble ? 



CHAPTEE VII 
THE SEA AND ITS SHORES 

We have already seen that the ocean is the goal at which the 
waste of the land arrives. The mantle of rock waste, creeping 
down slopes, is washed to the sea by streams, together with the 
material which the streams have worn from their beds and that 
dissolved by underground waters. In arid regions the winds 




FIG. 131. Sea Cliff and Rock Bench Cut in Chalk, Dover, England 

sweep waste either into bordering oceans or into more humid 
regions where rivers take it up and carry it on to the sea. 
Glaciers deliver the load of their moraines either directly to the 
sea or leave it for streams to transport to the same goal. All 
deposits made on the land, such as the flood plains of rivers, the 

155 



156 THE ELEMENTS OF GEOLOGY 

silts of lake beds, dune sands, and sheets of glacial drift, mark 
but pauses in the process which is to bring all the materials of 
the land now above sea level to rest upon the ocean bed. 

But the sea is also at work along all its shores as an agent 
of destruction, and we must first take up its work in erosion 
before we consider how it transports and deposits the waste of 
the land. 

SEA EROSION 

The sea cliff and the rock bench. On many coasts the land 
fronts the ocean in a line of cliffs (Fig. 131). To the edge of the 
cliffs there lead down valleys and ridges, carved by running 
water, which, if extended, would meet the water surface some 
way out from shore. Evidently they are now abruptly cut short 

at the present shore line because 
High tide ^ ie l an d has been cut back. 
--^, ^lerel Along the foot of the cliff lies a 

~~^ "ently shelving bench of rock, more 



FIG. 132. Diagram of Sea Cliff or less thickly veneered with sand 
ac, and Hock Bench rb &nd shingle> At low tide its imier 

The broken line indicates the mar or m i s laid bare, but at high tide 
former extent of the land 

it is covered wholly, and the sea 

washes the base of the cliffs. A notch, of which the sea cliff 
and the rock bench are the two sides, has been cut along the 
shore (Fig. 132). 

Waves. The position of the rock bench, with its inner margin 
slightly above low tide, shows that it has been cut by some 
agent which acts like a horizontal saw set at about sea level. 
This agent is clearly the surface agitation of the water; it is 
the wind-raised wave. 

As a wave comes up the shelving bench the crest topples 
forward and the wave " breaks," striking a blow whose force is 
measured by the momentum of all its tong of falling water 
(Fig. 133). On the coast of Scotland the force of the blows 



THE SEA AND ITS SHORES 



157 




struck by the waves of the heaviest storms has sometimes 
exceeded three tons to the square foot. But even a calm sea 
constantly chafes the shore. It heaves in gentle undulations 
known as the ground swell, the result of storms perhaps a 
thousand miles dis- 
tant, and breaks on 
the shore in surf. 

The blows of the 
waves are not struck 
with clear water 
only, else they 
would have little 
effect on cliffs of 
solid rock. Storm FlG - 133 ' Breakin g Wave , L *ke Superior 

waves arm themselves with the sand and gravel, the cobbles, 
and even the large bowlders which lie at the base of the cliff, 
and beat against it with these hammers of stone. 

Where a precipice descends sheer into deep water, waves swash up 
and down the face of the rocks but cannot break and strike effective 
blows. They therefore erode but little until the talus fallen from the 
cliff is gradually built up beneath the sea to the level at which the 
waves drag bottom upon it and break. 

Compare the ways in which different agents abrade. The wind 
lightly brushes sand and dust over exposed surfaces of rock. Running 
water sweeps fragments of various sizes along its channels, holding 
them with a loose hand. Glacial ice grinds the stones of its ground 
moraine against the underlying rock with the pressure of its enormous 
weight. The wave hurls fragments of rock against the sea cliff, bruising 
and battering it by the blow. It also rasps the bench as it drags sand 
and gravel to and fro upon it. 

Weathering of sea cliffs. The sea cliff furnishes the weapons 
for its own destruction. They are broken from it not only by 
the wave but also by the weather. Indeed the sea cliff weathers 
more rapidly, as a rule, than do rock ledges inland. It is abun- 
dantly wet with spray. Along its base the ground water of the 



158 THE ELEMENTS OF GEOLOGY 

neighboring land finds its natural outlet in springs which under- 
mine it. Moreover, it is unprotected by any shield of talus. 
Fragments of rock as they fall from its face are battered to 
pieces by the waves and swept out to sea. The cliff is thus 
left exposed to the attack of the weather, and its retreat would 
be comparatively rapid for this reason alone. 




EIG. 134. Sea Caves, La Jolla, California 
Copyright, 1899, by Detroit Photographic Company 

Sea cliffs seldom overhang, but commonly, as in Figure 134, slope sea- 
ward, showing that the upper portion has retreated at a more rapid 
rate than has the base. Which do you infer is on the whole the more 
destructive agent, weathering or the wave? 

Draw a section of a sea cliff cut in well jointed rocks whose joints 
dip toward the land. Draw a diagram of a sea cliff where the joints 
dip toward the sea. 

Sea caves. The wave does not merely batter the face of the cliff. 
Like a skillful quarryman it inserts wedges in all natural fissures, such 
as joints, and uses explosive forces. As a wave flaps against a crevice 
it compresses the air within with the sudden stroke ; as it falls back 
the air as suddenly expands. On lighthouses heavily barred doors have 
been burst outward by the explosive force of the air within, as it was 



THE SEA AND ITS SHORES 



159 



released from pressure when a partial vacuum was formed by theieflu- 

ence of the wave. Where a crevice is filled with water the entire force 

of the blow of the wave is transmitted by hydraulic pressure to the sides 

of the fissure. Thus storm 

waves little by little pry 

and suck the rock loose, 

and in this way, and by 

the blows which they strike 

with the stones of the 

beach, they quarry out 

about a joint, or wherever 

the rock may be weak, a 

recess known as a sea cave, 

provided that the rock 

above is coherent enough 

to form a roof. Otherwise 

FIG. 135. A Sea Arch, California 




Copyright, 1899, by Detroit Photographic 
Company 



an open chasm results. 

Blowholes and sea arches. 
As a sea cave is drilled back 
into the rock, it may encounter a joint or crevice opened to the surface 
by percolating water. The shock of the waves soon enlarges this to a 
blowhole, which one may find on the breezy upland, perhaps a hundred 
yards and more back from the cliff's edge. In quiet weather the blow- 
hole is a deep well; in 
storm it plays a fountain 
as the waves drive through 
the long tunnel below and 
spout their spray high in 
air in successive jets. As 
the roof of the cave thus 
breaks down in the rear, 
there may remain in front 
for a while a sea arch, 

similar to the natural 
FIG. 136. Chasms worn by Waves, . . ., . . , 

Coast of Scotland brid S es of land caverns 

(Fig. 135). 

Stacks and wave-cut islands. As the sea drives its tunnels and open 
drifts into the cliff, it breaks through behind the intervening portions 
and leaves them isolated as stacks, much as monuments are detached 





100 



THE SEA AND ITS SHORES 



161 




FIG. 138. Wave-Cut Islands, Scotland 
How far did the land once extend ? 



from inland escarpments by the weather ; and as the sea cliff retreats, 
these remnant masses may be left behind as rocky islets. Thus the 
rock bench is often set with stacks, islets in all stages of destruction, 
and sunken reefs, all 
wrecks of the land testi- 
fying to its retreat before 
the incessant attack of the 
waves. 

Coves. Where zones 
of soft or closely jointed 
rock outcrop along a 
shore, or where minor 
water courses come 
down to the sea and aid 
in erosion, the shore is 
worn back in curved reentrants called coves ; while the more 
resistant rocks on either hand are left projecting as headlands 
(Fig. 139). After coves are cut back a short distance by the 
waves, the headlands come to protect them, as with break- 
waters, and prevent their indefinite retreat. The shore takes a 
curve of equilibrium, along which the hard rock of the exposed 

headland and the weak rock 
of the protected cove wear 
back at an equal rate. 

Rate of recession. The 
rate at which a shore recedes 
depends on several factors. 
In soft or incoherent rocks 
exposed to violent storms 
the retreat is so rapid as to 
be easily measured. The coast of Yorkshire, England, whose 
cliffs are cut in glacial drift, loses seven feet a year on the 
average, and since the Norman conquest a strip a mile wide, 
with farmsteads and villages and historic seaports, has been 
devoured by the sea. The sandy south shore of Martha's 




FIG. 139. Coves formed in Softer Strata 
S, S ; while the Harder Strata H, H, 
are left as Headlands 



162 



THE ELEMENTS OF GEOLOGY 



Vineyard wears back three feet a year. But hard rocks retreat 
so slowly that their recession has seldom been measured by the 
records of history. 

SHORE DRIFT 

Bowlder and pebble beaches. About as fast as formed the 
waste of the sea cliff is swept both along the shore and out to 
sea. The road of waste along shore is the beach. We may also 
define the beach as the exposed edge of the sheet of sediment 




FIG. 140. A Pebble Beach, Cape Ann, Massachusetts 

formed by the carriage of land waste out to sea. At the foot of 
sea cliffs, where the waves are pounding hardest, one commonly 
finds the rock bench strewn on its inner margin with large 
stones, dislodged by the waves and by the weather and some- 
what worn on their corners and edges. From this bowlder beach 
the smaller fragments of waste from the cliff and the fragments 
into which the bowlders are at last broken drift on to more shel- 
tered places and there accumulate in a pebble beach, made of 
pebbles well rounded by the wear which they have suffered. 
Such beaches form a mill whose raw material is constantly 



THE SEA AND ITS SHORES 163 

supplied by the cliff. The breakers of storms set it in motion 
to a depth of several feet, grinding the pebbles together with 
a clatter to be heard above the roar of the surf. In such a rock 
crusher the life of a pebble is short. Where ships have stranded 
on our Atlantic coast with cargoes of hard-burned brick or of coal, 
a year of time and a drift of five miles along the shore have proved 
enough to wear brick and coal to powder. At no great distance 
from their source, therefore, pebble beaches give place to beaches 
of sand, which occupy the more sheltered reaches of the shore. 

Sand beaches. The angular sand grains of various minerals 
into which pebbles are broken by the waves are ground together 
under the beating surf and rounded, and those of the softer 
minerals are crushed to powder. The process, however, is a 
slow one, and if we study these sand grains under a lens we 
may be surprised to see that, though their corners and edges 
have been blunted, they are yet far from the spherical form 
of the pebbles from which they were derived. The grains are 
small, and in water they have lost about half their weight in 
air ; the blows which they strike one another are therefore weak. 
Besides, each grain of sand of the wet beach is protected by a 
cushion of water from the blows of its neighbors. 

The shape and size of these grains and the relative proportion 
of grains of the softer minerals which still remain give a rough 
measure of the distance in space and tune which they have 
traveled from their source. The sand of many beaches, derived 
from the rocks of adjacent cliffs or brought in by torrential 
streams from neighboring highlands, is dark with grains of a 
number of minerals softer than quartz. The white sand of other 
beaches, as those of the east coast of Florida, is almost wholly 
composed of quartz grains; for in its long travel down the 
Atlantic coast the weaker minerals have been worn to powder 
and the hardest alone survive. 

How does the absence of cleavage in quartz affect the durability of 
quartz sand? 



164 



THE ELEMENTS OF GEOLOGY 



How shore drift migrates. It is under the action of waves 
and currents that shore drift migrates slowly along a coast. 
Where waves strike a coast obliquely they drive the waste 
before them little by little along the shore. Thus on a north- 
south coast, where the predominant storms are from the north- 
east, there will be a migration of shore drift southwards. 

All shores are swept also by currents produced by winds and 
tides. These are usually far too gentle to transport of them- 
selves the coarse materials of which beaches are made. But 

while the wave stirs 
the grains of sand 
and gravel, and for 
a moment lifts them 
from the bottom, 
the current carries 
them a step forward 
011 their way. The 
current cannot lift 
and the wave can- 
not carry, but to- 
g e t h er the two 
transport the waste along the shore. The road of shore drift is 
therefore the zone of the breaking waves. 

The bay-head beach. As the waste derived from the wear 
of waves and that brought in by streams is trailed along a 
coast it assumes, under varying conditions, a number of dis- 
tinct forms. When swept into the head of a sheltered bay ft 
constitutes the bay-head beach. By the highest storm waves 
the beach is often built higher than the ground immediately 
behind it, and forms a dam inclosing a shallow pond or 
marsh. 

The bay bar. As the stream of shore drift reaches the mouth 
of a bay of some size it often occurs that, instead' of turning in, 
it sets directly across toward the opposite headland. The waste 




FIG. 141. A Bay Bar, Lake Ontario 



THE SEA AND ITS SHORES 



165 







is carried out from shore into the deeper waters of the bay 
mouth, where it is no longer supported by the breaking waves, 
and sinks to the bottom. The dump is gradually built to the 
surface as a stubby spur, pointing across the bay, and as it 
reaches the zone of wave action current and wave can now 
combine to carry shore drift along it, depositing their load con- 
tinually at the point of the spur. An embankment is thus con- 
structed in much the 
same manner as a rail- 
way fill, which, while 
it is building, serves as 
a roadway along which 
the dirt from an ad- 
jacent cut is carted to 
be dumped at the end. 

When the embankment is completed it bridges the bay with 
a highway along which shore drift now moves without inter- 
ruption, and becomes a bay bar. 

Incomplete bay bars. Under certain conditions the sea can- 
not carry out its intention to bridge a bay. Rivers discharging 
in bays demand open way to the ocean. Strong tidal currents 
also are able to keep open channels scoured by their ebb and 
flow. In such cases the most that land waste can do is to build 
spits .and shoals, narrowing and shoaling the channel as much 
as possible. Incomplete bay bars sometimes have their points 

recurved by currents setting at 
right angles to the stream of shore 
FIG. 143. Cross Section of Sand drift, and are then classified as 

Reef sr, and Lagoon ; sZ, Sea hooks (Fig. 142). 

Sand reefs. On low coasts 

where shallow water extends some distance out, the highway of 
shore drift lies along a low, narrow ridge, termed the sand reef, 
separated from the land by a narrow stretch of shallow water 
called the lagoon (Fig. 143). At intervals the reef is held open 



166 



THE ELEMENTS OF GEOLOGY 



f 



by inlets, gaps through which the tide flows and ebbs, and by 
which the water of streams finds way to the sea. 

No finer example of this kind of shore line is to be found in the 
world than the coast of Texas. From near the mouth of the Rio 
Grande a continuous sand reef draws its even 
curve for a hundred miles to Corpus Christi 
Pass, and the reefs are but seldom interrupted 
by inlets as far north as Galveston Harbor. 
On this coast the tides are variable and ex- 
ceptionally weak, being less than one foot in 
height, while the amount of waste swept along 
the shore is large. The lagoon is extremely 
shallow, and much of it is a mud flat too shoal 
for even small boats. On the coast of New 
Jersey strong tides are able to keep open inlets 
at intervals of from two to twenty miles in 
spite of a heavy alongshore drift. 



Sand reefs are formed where the water 
is so shallow near shore that storm waves 
cannot run in it and therefore break some 
distance out from land. Where storm 
waves first drag bottom they erode and 
deepen the sea floor, and sweep in sedi- 
ment as far as the line where they break. 
Here, where they lose their force, they 
drop their load and beat up the ridge 




FIG. 144. Sand Reef 
and Lagoon, Texas 



which is known as the sand reef when it reaches the surface. 



SMOKES OF ELEVATION AND DEPRESSION 

Our studies have already brought to our notice two distinct 
forms of strand lines, one the high, rocky coast cut back to 
cliffs by the attack of the waves, and the other the low, sandy 
coast where the waves break usually upon the sand reef. To 
understand the origin of these two types we must know that 



THE SEA AND ITS SHORES 167 

the meeting place of sea and land is determined primarily by 
movements of the earth's crust. Where a coast land emerges 
the shore line moves seaward; where it is being submerged 
the shore line advances on the land. 

Shores of elevation. The retreat of the sea, either because of 
a local uplift of the land or for any other reason, such as the 
lowering of any portion of ocean bottom, lays bare the inner 
margin of the sea floor. Where the sea floor has long received 
the waste of the land it has been built up to a smooth, subaque- 
ous plain, gently shelving from the land. Since the new shore 
line is drawn across this even surface it is simple and regular, 
and is bordered on the one side by shallow water gradually 
deepening seaward, and on the other by low land composed of 
material which has not yet thoroughly consolidated to firm rock. 
A sand reef is soon beaten up by the waves, and for some time 
conditions will favor its growth. The loss of sand driven into 
the lagoon beyond, and of that ground to powder by the surf 
and carried out to sea, is more than made up by the stream of 
alongshore drift, and especially by the drag of sediments to the 
reef by the waves as they deepen the sea floor on its seaward side. 

Meanwhile the lagoon gradually fills with waste from the 
reef and from the land. It is invaded by various grasses and 
reeds which have learned to grow in salt and brackish water; 
the marsh, laid bare only at low tide, is built above high tide 
by wind drift and vegetable deposits, and becomes a meadow, 
soldering the sand reef to the mainland. 

While the lagoon has been filling, the waves have been so 
deepening the sea floor off the sand reef that at last they are 
able to attack it vigorously. They now wear it back, and, driving 
the shore line across the lagoon or meadow, cat a line of low 
cliffs on the mainland. Such a shore is that of Gascony in 
southwestern France, a low, straight, sandy shore, bordered 
by dunes and unprotected by reefs from the attack of the waves 
of the Bay of Biscay. 



168 



THE ELEMENTS OF GEOLOGY 



We may say, then, that on shores of elevation the presence 
of sand reefs and lagoons indicates the stage of youth, while 

the absence of these 
features and the 
vigorous and unim- 
peded attack by the 
sea upon the mam- 
land indicate the 
stage of maturity. 
Where much waste 
is brought in by 
rivers the maturity 
of such a coast may 
be long delayed. 
The waste from the 
land keeps the sea 
shallow offshore and 
constantly renews 
the sand reef. The 
energy of the waves 
is consumed in hand- 
ling shore drift, and 
no energy is left for 
an effective attack 
upon the land. In- 
deed, with an exces- 

FIG. 145. Map of New Jersey, with that Portion s i ve amount of waste 
of the State one Hundred Feet and more above , . , 

Sea Level shaded brought down by 

Describe the coast line which the state would have if streamst heiaiia 
depressed one hundred feet. Compare it with the may be built Out and 

present coast line encroach tempo- 

rarily upon the sea ; and not until long denudation has lowered 
the land, and thus decreased the amount of waste from it, may 
the waves be able to cut through the sand reef and thus the 
coast reach maturity. 




THE SEA AND ITS SHORES 



169 



SHORES OF DEPRESSION 

Where a coastal region is undergoing submergence the shore 
line moves landward. The horizontal plane of the sea now 
intersects an old land surface roughened by subaerial denuda- 
tion. The shore line is irregular and indented in proportion to 
the relief of the land and the amount 
of the submergence which the land 
has suffered. It follows up partially 
submerged valleys, forming bays, and 
bends round the divides, leaving 
them to project as promontories and 
peninsulas. The outlines of shores 
of depression are as varied as are 
the forms of the land partially sub- 
merged. We give a few typical illus- 
trations. 



The characteristics of the coast of 
Maine are due chiefly to the fact that a 
mountainous region of hard rocks, once 
worn to a peneplain, and after a sub- 
sequent elevation deeply dissected by 
north-south valleys, has subsided, the 
depression amounting on its southern 
margin to as much as six hundred feet 
below sea level. Drowned valleys pene- 
trate the land in long, narrow bays, and 
rugged divides project in long, narrow 
land arms prolonged seaward by islands 




FIG. 146. Chesapeake Bay 

Draw a sketch map of this area 
before its depression 



representing the high portions of their extremities. Of this exceedingly 
ragged shore there are said to be two thousand miles from the Xew 
Brunswick boundary as far west as Portland, a straight-line distance 
of but two hundred miles. Since the time of its greatest depression 
the land is known to have risen some three hundred feet ; for the bays 
have been shortened, and the waste with which their floors were strewn 
is now in part laid bare as clay plains about the bay heads and in 
narrow selvages about the peninsulas and islands. 



170 THE ELEMENTS OF GEOLOGY 

The coast of Dalmatia, on the Adriatic Sea, is characterized by long 
land arms and chains of long and narrow islands, all parallel to the 
trend of the coast. A region of parallel mountain ranges has been 
depressed, and the longitudinal valleys which lie between them are 
occupied by arms of the sea. 

Chesapeake Bay is a branching bay due to the depression of an 
ancient coastal plain which, after having emerged from the sea, was 
channeled with broad, shallow valleys. The sea has invaded the valley 
of the trunk stream and those of its tributaries, forming a shallow bay 
whose many branches are all directed toward its axis (Fig. 146). 

Hudson Bay, and the North, the Baltic, and the Yellow seas are 
examples where the sinking of the land has brought the sea in over 
low plains of large extent, thus deeply indenting the continental out- 
line. The rise of a few hundred feet would restore these submerged 
plains to the land. 

The cycle of shores of depression. In its infantile stage the 
outline of a shore of depression depends almost wholly on the 
previous relief of the land, and but little on erosion by the sea. 
Sea cliffs and narrow benches appear where headlands and 
outlying islands have been nipped by the waves. As yet, little 
shore waste has been formed. The coast of Maine is an example 
of this stage. 

In early youth all promontories have been strongly cliffed, 
and under a vigorous attack of the sea the shore of open bays 
may be cut back also. Sea stacks and rocky islets, caves and 
coves, make the shore minutely ragged. The irregularity of the 
coast, due to depression, is for a while increased by differential 
wave wear on harder and softer rocks. The rock bench is still 
narrow. Shore waste, though being produced in large amounts, 
is for the most part swept into deeper water and buried out 
of sight. Examples of this stage are the east coast of Scotland 
and the California coast near San Francisco. 

Later youtli is characterized by a large accumulation of 
shore waste. The rock bench has been cut back so that it 
now furnishes a good roadway for shore drift. The stream of 



THE SEA AXD ITS SHORES 



171 



alongshore drift grows larger and larger, filling the heads of the 
smaller bays with beaches, building spits and hooks, and tying 
islands with sand bars to the mainland. It bridges the larger 
bays with bay bars, while their length is being reduced as their 
inclosing promontories are cut back by the waves. Thus there 
comes to be a straight, continuous, and easy road, no longer 
interrupted by headlands and bays, for the transportation of 
waste alongshore. 
The Baltic coast of 
Germany is in this 
stage. 

All this while 
streams have been 
busy filling with 
delta deposits the 
bays into which 
they empty. By 
these steps a coast 
gradually advances 
to maturity, the 
stage when the irregularities due to depression have been 
effaced, when outlying islands formed by subsidence have been 
planed away, and when the shore line has been driven back 
behind the former bay heads. The sea now attacks the land 
most effectively along a continuous and fairly straight line of 
cliffs. Although the first effect of wave wear was to increase 
the irregularities of the shore, it sooner or later rectifies it, 
making it simple and smooth. Northwestern France may be 
cited as an upland plain, dissected and depressed, whose coast 
has reached maturity (Fig. 147). 

In the old age of coasts the rock bench is cut back so far that 
the waves can no longer exert their full effect upon the shore. 
Their energy is dissipated in moving shore drift hither and 
thither and in abrading the bench when they drag bottom 




FIG. 147. Portion of the Northwest Coast of France 



172 



THE ELEMENTS OF GEOLOGY 



upon it. Little by little the bench is deepened by tidal currents 
and the drag of waves ; but this process is so slow that mean- 
while the sea cliffs melt down under the weather, and the 
bench becomes a broad shoal where waves and tides gradually 
work over the waste from the land to greater fineness and sweep 
it out to sea. 

Plains of marine abrasion. While subaerial denudation 
reduces the land to baselevel, the sea is sawing its edges to 




K 



FIG. 148. The South Shore of Martha's Vineyard 

The land is shaded. To what class of coasts does this belong? What stage 
has it reached, and by what process ? What changes will take place in 
the future ? Draw map showing this coast at beginning of cycle. 

wave base, i.e. the lowest limit of the wave's effective wear. The 
widened rock bench forms when uplifted a plain of marine 
abrasion, which like the peneplain bevels across strata regardless 
of their various inclinations and various degrees of hardness. 

How may a plain of marine abrasion be expected to differ from a 
peneplain in its mantle of waste ? 

Compared with subaerial denudation, marine abrasion is a 

comparatively feeble agent. At the rate of five feet per century 

a higher rate than obtains on the youthful rocky coast of 

Britain it would require more than ten million years to pare 



THE SEA AND ITS SHORES 173 

a strip one hundred miles wide from the margin of a conti- 
nent, a time sufficient, at the rate at which the Mississippi 
valley is now being worn away, for subaerial denudation to 
lower the lands of the globe to the level of the sea. 

Slow submergence favors the cutting of a wide rock bench. 
The water continually deepens upon the bench; storm waves 
can therefore always ride in to the base of the cliff's and attack 
them with full force; shore waste cannot impede the onset 
of the waves, for it is continually washed out in deeper water 
below wave base. 

Basal conglomerates. As the sea marches across the land 
during a slow submergence, the platform is covered with sheets 
of sea-laid sediments. Lowest of these is a conglomerate, 
the bowlder and pebble beach, widened indefinitely by the 
retreat of the cliffs at whose base it was formed, and preserved 
by the finer deposits laid upon it in the constantly deepening 
water as the land subsides. Such basal conglomerates are not 
uncommon among the ancient rocks of the land, and we may 
know them by their rounded pebbles and larger stones, com- 
posed of the same kind of rock as that of the abraded and 
evened surface on which they lie. 



CHAPTEE VIII 
OFFSHORE AND DEEP-SEA DEPOSITS 

The alongshore deposits which we have now studied are 
the exposed edge of a vast subaqueous sheet of waste which 
borders the continents and extends often for as much as two 
or three hundred miles from land. Soundings show that off- 
shore deposits are laid in belts parallel to the coast, the coarsest 
materials lying nearest to the land and the finest farthest out. 
The pebbles and gravel and the clean, coarse sand of beaches 
give place to broad stretches of sand, which grows finer and 
finer until it is succeeded by sheets of mud. Clearly there is 
an offshore movement of waste by which it is sorted, the 
coarser being sooner dropped and the finer being carried 
farther out. 

OFFSHORE DEPOSITS 

The debris torn by waves from rocky shores is far less in 
amount than the waste of the land brought down to the sea 
by rivers, being only one thirty-third as great, according to a 
conservative estimate. Both mingle alongshore in all the forms 
of beach and bar that have been described, and both are together 
slowly carried out to sea. On the shelving ocean floor waste is 
agitated by various movements of the unquiet water, by the 
undertow (an outward-running bottom current near the shore), 
by the ebb and flow of tides, by ocean currents where they 
approach the land, and by waves and ground swells, whose 
effects are sometimes felt to a depth of six hundred feet. By 
all these means the waste is slowly washed to and fro, and as 
it is thus ground finer and finer and its soluble parts are more 

174 



OFFSHORE AND DEEP-SEA DEPOSITS 175 

and more dissolved, it drifts farther and farther out from land 
It is by no steady and rapid movement that waste is swept 
from the shore to its final resting place. Day after day and 
century after century the grains of sand and particles of mud 
are shifted to and fro, winnowed and spread in layers, which are 
destroyed and rebuilt again and again before they are buried 
safe from further disturbance. 

These processes which are hidden from the eye are among 
the most important of those with which our science has to do ; 
for it is they which have given shape to by far the largest part 
of the stratified rocks of which the land is made. 

The continental delta. This fitting term has been recently 
suggested for the sheet of waste slowly accumulating along the 
borders of the continents. Within a narrow belt, which rarely 
exceeds two or three hundred miles, except near the mouths 
of muddy rivers such as the Amazon and Congo, nearly all the 
waste of the continent, whether worn from its surface by the 
weather, by streams, by glaciers, or by the wind, or from its 
edge by the chafing of the waves, comes at last to its final 
resting place. The agencies which spread the material of the 
continental delta grow more and more feeble as they pass into 
deeper and more quiet water away from shore. Coarse materials 
are therefore soon dropped along narrow belts near land. Gravels 
and coarse sands lie in thick, wedge-shaped masses which thin 
out seaward rapidly and give place to sheets of finer sand. 

Sea muds. Outermost of the sediments derived from the waste 
of the continents is a wide belt of mud ; for fine clays settle so 
slowly, even in sea water, whose saltness causes them to sink 
much faster than they would in fresh water, that they are 
wafted far before they reach a bottom where they may remain 
undisturbed. Muds are also found near shore, carpeting the 
floors of estuaries, and among stretches of sandy deposits in 
hollows where the more quiet water has permitted the finer 
silt to rest. 



176 THE ELEMENTS OF GEOLOGY 

Sea muds are commonly bluish and consolidate to bluish 
shales ; the red coloring matter brought from land waste iron 
oxide is altered to other iron compounds by decomposing 
organic matter in the presence of sea water. Yellow and red 
muds occur where the amount of iron oxide in the silt brought 
down to the sea by rivers is too great to be reduced, or decom- 
posed, by the organic matter present. 

Green muds and green sand owe their color to certain chem- 
ical changes which take place where waste from the land accu- 
mulates on the sea floor with extreme slowness. A greenish 

O 

mineral called glauconite a silicate of iron and alumina is 
then formed. Such deposits, known as green sand, are now in 
process of making in several patches off the Atlantic coast, and 
are found on the coastal plain of New Jersey among the off- 
shore deposits of earlier geological ages. 

Organic deposits. Living creatures swarm along the shore and 
on the shallows out from land as nowhere else in the ocean. 
Seaweed often mantles the rock of the sea cliff between the 
levels of high and low tide, protecting it to some degree from 
the blows of waves. On the rock bench each little pool left 
by the ebbing tide is an aquarium abounding in the lowly 
forms of marine life. Below low-tide level occur beds of mol- 
luscous shells, such as the oyster, with countless numbers of 
other humble organisms. Their harder parts the shells of 
inollusks, the white framework of corals, the carapaces of crabs 
and other crustaceans, the shells of sea urchins, the bones and 
teeth of fishes are gradually buried within the accumulating 
sheets of sediment, either whole or, far more often, broken into 
fragments by the waves. 

By means of these organic remains each layer of beach 
deposits and those of the continental delta may contain a record 
of the life of the time when it was laid. Such a record has 
been made ever since living creatures with hard parts appeared 
upon the globe. We shall find it sealed away in the stratified 



OFFSHORE AND DEEP-SEA DEPOSITS 



177 



rocks of the continents, parts of ancient sea deposits now 
raised to form the dry land. Thus we have in the traces of 
living creatures found in the rocks, i.e. in fossils, a history of the 
progress of life upon the planet. 

Molluscous shell deposits. The forms of marine life of impor- 
tance in rock making thrive best in clear water, where little 
sediment is being laid, and where at the same time the depth is 



FIG. 149. Coquma, Florida 




not so great as to deprive them of needed light, heat, and of 
sufficient oxygen absorbed by sea water from the air. In such 
clear and comparatively shallow water there often grow count- 
less myriads of animals, such as mollusks and corals, whose 
shells and skeletons of carbonate of lime gradually accumulate 
in beds of limestone. 

A shell limestone made of broken fragments cemented together is 
sometimes called coquina, a local term applied to such beds recently 
uplifted from the sea along the coast of Florida (Fig. 149). 



178 THE ELEMENTS OF GEOLOGY 

Oolitic limestone (oon, an egg ; lithos, a stone) is so named from the 
likeness of the tiny spherules which compose it to the roe of fish. 
Corals and shells have been pounded by the waves to calcareous sand, 
and each grain has been covered with successive concentric coatings of 
lime carbonate deposited about it from solution. 

The impalpable powder to which calcareous sand is ground 
by the waves settles at some distance from shore in deeper and 
quieter water as a limy silt, and hardens into a dense, fine- 
grained limestone in which perhaps no trace of fossil is found 
to suggest the fact that it is of organic origin. 

From Florida Keys there extends south to the trough of Florida 
Straits a limestone bank covered by from five hundred and forty to 
eighteen hundred feet of water. The rocky bottom consists of lime- 
stone now slowly building from the accumulation of the remains of 
mollusks, small corals, sea urchins, worms with calcareous tubes, and 
lime-secreting seaweed, which live upon its surface. 

Where sponges and other silica-secreting organisms abound on 
limestone banks, silica forms part of the accumulated deposit, 
either in its original condition, as, for example, the spicules of 
sponges, or gathered into concretions and layers of flint. 

Where considerable mud is being deposited along with car- 
bonate of lime there is in process of making a clayey limestone 
or a limy shale ; where considerable sand, a sandy limestone or 
a limy sandstone. 

Consolidation of offshore deposits. We cannot doubt that all 
these loose sediments of the sea floor are being slowly consoli- 
dated to solid rock. They are soaked with water which carries 
in solution lime carbonate and other cementing substances. 
These cements are deposited between the fragments of shells 
and corals, the grains of sand and the particles of mud, binding 
them together into firm rock. Where sediments have accumu- 
lated to great thickness the lower portions tend also to consol- 
idate under the weight of the overlying beds. Except in the 
case of limestones, recent sea deposits uplifted to form land are 




179 



180 



THE ELEMENTS OF GEOLOGY 



seldom so well cemented as are the older strata, which have 
long been acted upon by underground waters deep below the 
surface within the zone of cementation, and have- been exposed 
to view by great erosion. 

Ripple marks, sun cracks, etc. The pulse of waves and tidal 
currents agitates the loose material of offshore deposits, throw- 
ing it into fine parallel ridges called ripple marks. One may 

see this beautiful 
ribbing imprinted 
on beach sands un- 
covered by the out- 
going tide, and it is 
also produced where 
the water is of con- 
siderable depth. 
While the tide is 
out the surface of 
shore deposits may 
be marked by the 
footprints of birds 
and other animals, 
or by the raindrops 
of a passing shower 
(Fig. 153). The mud of flats, thus exposed to the sun and dried, 
cracks in a characteristic way (Figs. 151 and 152). Such mark- 
ings may be covered over with a thin layer of sediment at the 
next flood tide and sealed away as a lasting record of the manner 
and place in which the strata were laid. In Figure 150 we have 
an illustration of a very ancient ripple-marked sand consolidated 
to hard stone, uplifted and set on edge by movements of the 
earth's crust, and exposed to open air after long erosion. 

Stratification. For the most part the sheet of sea-laid waste 
is hidden from our sight. Where its edge is exposed along the 
shore we may see the surface markings which have just been 




FIG. 151. Sun Cracks 



OFFSHORE AND DEEP-SEA DEPOSITS 



181 



noticed. Soundings also, and the observations made in shallow 
waters by divers, tell something of its surface; but to learn 
more of its structures 
we must study those 
ancient sediments which 
have been lifted from 
the sea and dissected 
bysubaerial agencies. 
From them we ascertain 
that sea deposits are 
stratified. They lie in 
distinct layers which 
often differ from one an- 
other in thickness, in 
size of particles, and 
perhaps in color. They 
are parted by bedding 
planes, each of which 
represents either a 
change in material or FIG. 152. The Under Side of a Layer de- 
a pause during which posited upon a Sun-Cracked Surface, 

... showing Casts of the Cracks 

deposition ceased and 

the material of one layer had tune to settle and become some- 
what consolidated before the material of the next was laid 
upon it. Stratification is thus due to intermittently acting 
forces, such as the agitation of the 
water during storms, the flow and ebb 
of the tide, and the shifting channels 
of tidal currents. Off the mouths of 
rivers, stratification is also caused by 
the coarser and more abundant material 
brought down at time of floods being 
laid on the finer silt which is dis- 
Fio. 153. Rain Prints charged during ordinary stages. 





182 



THE ELEMENTS OF GEOLOGY 



How stratified deposits are built up is well illustrated in the flats 
which border estuaries, such as the Bay of Fundy. Each advance of 
the tide spreads a film of mud, which dries and hardens in the air 
during low water before another film is laid upon it by the next 
incoming tidal flood. In this way the flats have been covered by a clay 
which splits into leaves as thin as sheets of paper. 

It is in fine material, such as clays and shales and limestones, 
that the thinnest and most uniform layers, as well as those 
of widest extent, occur. On the other hand, coarse materials 
are commonly laid in thick beds, which soon thin out seaward 




FIG. 154. Cross Bedding in Sandstone, England 



to deposits of 
finer stuff. In 
a general way 
strata are laid 
in well-nigh 
horizontal 
sheets, for the 
surface on 
which they are 
laid is generally 
of very gentle 

inclination. Each stratum, however, is lenticular, or lenslike, in 
form, having an area where it is thickest, and thinning out thence 
to its edges, where it is overlapped by strata similar in shape. 

Cross bedding. There is an apparent exception to this rule where 
strata whose upper and lower surfaces may be about horizontal are 
made up of layers inclined at angles which may be as high as the 
angle of repose. In this case each stratum grew by the addition along 
its edge of successive layers of sediment, precisely as does a sand bar in 
a river, the sand being pushed continuously over the edge and coming 
to rest on a sloping surface. Shoals built by strong and shifting tidal 
currents often show successive strata in which the cross bedding is 
inclined in different directions. 



OFFSHORE AND DEEP-SEA DEPOSITS 183 

Thickness of sea deposits. Eemembering the vast amount 
of material denuded from the land and deposited offshore, we 
should expect that with the lapse of time sea deposits would 
have grown to an enormous thickness. It is a suggestive fact 
that, as a rule, the profile of the ocean bed is that of a soup 
plate, a basin surrounded by a flaring rim. On the continen- 
tal shelf, as the rim is called, the water is seldom more than 
six hundred feet in depth at the outer edge, and shallows grad- 
ually towards shore. Along the eastern coast of the United 
States the continental shelf is from fifty to one hundred and 
more miles in width ; on the Pacific coast it is much narrower. 
So far as it is due to upbuilding, a wide continental shelf, such 
as that of the Atlantic coast, implies a massive continental 
delta thousands of feet in thickness. The coastal plain of the 
Atlantic states may be regarded as the emerged inner margin 
of this shelf, and borings made along the coast probe it to the 
depth of as much as three thousand feet without finding the 
bottom of ancient offshore deposits. Continental shelves may 
also be due in part to a submergence of the outer margin of 
a continental plateau and to marine abrasion. 

Deposition of sediments and subsidence. The stratified rocks 
of the land show in many places ancient sediments which reach 
a thickness which is measured hi miles, and which are yet the 
product of well-nigh continuous deposition. Such strata may 
prove by their fossils and by their composition and structure 
that they were all laid offshore in shallow water. We must infer 
that, during the vast length of time recorded by the enormous 
pile, the floor of the sea along the coast was slowly sinking, 
and that the trough was constantly being filled, foot by foot, 
as fast as it was depressed. Such gradual, quiet movements of 
the earth's crust not only modify the outline of coasts, as 
we have seen, but are of far greater geological importance in 
that they permit the making of immense deposits of stratified 
rock 



184 THE ELEMENTS OF GEOLOGY 

A slow subsidence continued during long time is recorded 
also in the succession of the various kinds of rock that come 
to be deposited in the same area, As the sea transgresses the 
land, i.e. encroaches upon it, any given part of the sea bottom 
is brought farther and farther from the shore. The basal con- 
glomerate formed by bowlder and pebble beaches comes to be 
covered with sheets of sand, and these with layers of mud as the 
sea becomes deeper and the shore more remote ; while -deposits 
of limestone are made when at last no waste is brought to the 



&ea level 




FIG. 155. Succession of Deposits recording a Transgressing Sea 
c, conglomerate ; ss, sandstone ; sh, shale ; Im, limestone 

place from the now distant land, and the water is left clear for 
the growth of mollusks and other lime-secreting organisms. 

Rate of deposition. As deposition in the sea corresponds to 
denudation on the land, we are able to make a. general estimate 
of the rate at which the former process is going on. Leaving 
out of account the soluble matter removed, the Mississippi is 
lowering its basin at the rate of one foot in five thousand years, 
and we may assume this as the average rate at which the 
earth's land surface of fifty-seven million square miles is now 
being denuded by the removal of its mechanical waste. But sedi- 
ments from the land are spread within a zone but two or three 
hundred miles in width along the margin of the continents, a 
line one hundred thousand miles long. As the area of deposi- 
tion about twenty-five million square miles is about one 
half the area of denudation, the average rate of deposition must 
be twice the average rate of denudation, i.e. about one foot in 
twenty-five hundred years. If some deposits are made much 
more rapidly than this, others are made much more slowly. If 



OFFSHORE AND DEEP-SEA DEPOSITS 



185 




they were laid no faster than the present average rate, the strata 
of ancient sea deposits exposed in a quarry fifty feet deep repre- 
sent a lapse of at least one hundred and twenty-five thousand 
years', and those of a formation five hundred feet thick required 
for their accumulation one million two hundred and fifty thou- 
sand years. 

The sedimentary record and the denudation cycle. We have 
seen that the successive stages in a cycle of denudation, such as 
that by which a land mass 
of lofty mountains is worn 
to low plains, are marked 
each by its own peculiar 
land forms, and that the 
forms of the earlier stages 
are more or less completely 
effaced as the cycle draws 
toward an end. Far more 
lasting records of each stage 
are left in the sedimentary deposits of the continental delta. 
Thus, in the youth of such a land mass as we have mentioned, 
torrential streams flowing down the steep mountain sides de- 
liver to the adjacent sea their heavy loads of coarse waste, and 
thick offshore deposits of sand and gravel (Fig. 156) record 
the high elevation of the bordering land. As the land is worn 
to lower levels, the amount and coarseness of the w r aste 
brought to the sea diminishes, until the sluggish streams carry 
only a fine silt which settles on the ocean floor near to land in 
wide sheets of mud which harden into shale. At last, in the 
old age of the region (Fig. 157), its low plains contribute little 
to the sea except the soluble elements of the rocks, and in the 
clear waters near the land lime-secreting organisms flourish and 
their remains accumulate in beds of limestone. When long- 
weathered lands mantled with deep, well-oxidized waste are 
uplifted by a gradual movement of the earth's crust, and the 



FIG. 156. Thick Offshore Deposits of 
Coarse Waste recording the Presence 
of a Young Mountain Range near 
Shore 



186 THE ELEMENTS OF GEOLOGY 

mantle is rapidly stripped off by the revived streams, the uprise 
is recorded in wide deposits of red and yellow clays and sands 
upon the adjacent ocean floor. 

Where the waste brought in is more than the waves can 
easily distribute, as off the mouths of turbid rivers which drain 
highlands near the sea, deposits are little winnowed, and are laid 
in rapidly alternating, shaly sandstones and sandy shales. 

Where the highlands are of igneous rock, such as granite, 
and mechanical disintegration is going on more rapidly than 
chemical decay, these conditions are recorded in the nature of 



/Sea level 




FIG. 157. Offshore Deposits recording the Old Age of the 
Adjacent Land 

ss, sandstone; .s7i, shale; Im, limestone 

the deposits laid offshore. The waste swept in by streams con- 
tains much feldspar and other minerals softer and more soluble 
than quartz, and where the waves have little opportunity to 
wear and winnow it, it comes to rest in beds of sandstone in 
which grains of feldspar and other soft minerals are abundant. 
Such feldspathic sandstones are known as arkose. 

On the other hand, where the waste supplied to the sea comes 
chiefly from wide, sandy, coastal plains, there are deposited off- 
shore clean sandstones of well-worn grains of quartz alone. In 
such coastal plains the waste of the land is stored for ages. 
Again and again they are abandoned and invaded by the sea as 
from time to time the land slowly emerges and is again sub- 
merged. Their deposits are long exposed to the weather, and 
sorted over by the streams, and winnowed and worked over 
again and again by the waves. In the course of long ages such 
deposits thus become thoroughly sorted, and the grains of all 
minerals softer than quartz are ground to mud. 




OFFSHORE AND DEEP-SEA DEPOSITS 187 

DEEP-SEA OOZES AND CLAYS 

Globigerina ooze. Beyond the reach of waste from the land 
the bottom of the deep sea is carpeted for the most part with 
either chalky ooze or a fine red clay. The surface waters of 
the warm seas swarm with minute and lowly animals belong- 
ing to the order of the Foraminif- 
era, which secrete shells of carbonate 
of lime. At death these tiny white 
shells fall through the sea water like 
snowflakes in the air, and, slowly dis- 
solving, seem to melt quite away be- 
fore they can reach depths greater 
than about three miles. Near shore 
they reach bottom, but are masked 
by the rapid deposit of waste derived 
from the land. At intermediate FIG. 158. Globigerina Ooze 
i ,T ,1 ,i ,1 n under the Microscope 

depths they mantle the ocean floor 

with a white, soft lime deposit known as Globigerina ooze, from 
a genus of the Foraminifera which contributes largely to its 
formation. 

Red clay. Below depths of from fifteen to eighteen thousand 
feet the ocean bottom is sheeted with red or chocolate colored 
clay. It is the insoluble residue of seashells, of the debris 
of submarine volcanic eruptions, of volcanic dust wafted by the 
winds, and of pieces of pumice drifted by ocean currents far 
from the volcanoes from which they were hurled. The red 
clay builds up with such inconceivable slowness that the teeth 
of sharks and the hard ear bones of whales may be dredged in 
large numbers from the deep ocean bed, where they have lain 
unburied for thousands of years ; and an appreciable part of the 
clay is also formed by the dust of meteorites consumed in the 
atmosphere, a dust which falls everywhere on sea and land, 
but which elsewhere is wholly masked by other deposits. 



188 THE ELEMENTS OF GEOLOGY 

The dark, cold abysses of the ocean are far less affected by 
change than any other portion of the surface of the lithosphere. 
These vast, silent plains of ooze lie far below the reach of 
storms. They know no succession of summer and winter, or of 
night and day. A mantle of deep and quiet water protects them 
from the agents of erosion which continually attack, furrow, and 
destroy the surface of the land. While the land is the area of 
erosion, the sea is the area of deposition. The sheets of sedi- 
ment which are slowly spread there tend to efface any inequal- 
ities, and to form a smooth and featureless subaqueous plain. 

With few exceptions, the stratified rocks of the land are 
proved by their fossils and composition to have been laid in 
the sea; but in the same way they are proved to be offshore, 
shallow-water deposits, akin to those now making on continen- 
tal shelves. Deep-sea deposits are absent from the rocks of the 
land, and we may therefore infer that the deep sea has never 
held sway where the continents now are, that the continents 
have ever been, as now, the elevated portions of the lithosphere, 
and that the deep seas of the present have ever been its most 
depressed portions. 

THE KEEF-BUILDING CORALS 

In warm seas the most conspicuous of rock-making organisms 
are the corals known as the reef builders. Floating in a boat 
over a coral reef, as, for example, off the south coast of Florida 
or among the Bahamas, one looks down through clear water on 
thickets of branching coral shrubs perhaps as much as eight 
feet high, and hemispherical masses three or four feet thick, all 
abloom with countless minute flowerlike coral polyps, gorgeous 
in their colors of yellow, orange, green, and red. In structure 
each tiny polyp is little more than a fleshy sac whose mouth 
is surrounded with petal-like tentacles, or feelers. From the 
sea water the polyps secrete calcium carbonate and build it up 
into the stony framework which supports their colonies. Boring 



OFFSHORE AND DEEP-SEA DEPOSITS 189 

mollusks, worms, and sponges perforate and honeycomb this 
framework even while its surface is covered with myriads of 
living polyps. It is thus easily broken by the waves, and white 
fragments of coral trees strew the ground beneath. Brilliantly 
colored fishes live in these coral groves, and countless mollusks, 
sea urchins, and other forms of marine life make here their 




FIG. 159. Patch of Growing Corals exposed at an Exceptionally Low 
Tide, Great Barrier Reef, Australia 

home. With the debris from all these sources the reef is con- 
stantly built up until it rises to low-tide level. Higher than this 
the corals cannot grow, since they are killed by a few hours' 
exposure to the air. 

When the reef has risen to wave base, the waves abrade it 
on the windward side and pile to leeward coral blocks torn 
from their foundation, filling the interstices with finer fragments. 
Thus they heap up along the reef low, narrow islands (Fig. 160). 



190 THE ELEMENTS OF GEOLOGY 

Keef building is a comparatively rapid progress. It has been 
estimated that off Florida a reef could be built up to the surface 
from a depth of fifty feet in about fifteen hundred years. 

Coral limestones. Limestones of various kinds are due to the 

reef builders. The reef rock is made of corals in place and 

broken fragments of all sizes, cemented together with calcium 

carbonate from solution by infiltrating waters. On the island 

beaches coral sand is forming oolitic 

r'^r^^^^f^^ limestone, and the white coral mud 

with which the sea is milky for miles 
FIG. 160. Wave-Built Island . 

on Coral Reef about the reef in times of storm settles 

and concretes into a compact limestone 

r, reef ; s, sea level 

of finest grain. Corals have been 

among the most important limestone builders of the sea ever 
since they made their appearance in the early geological ages. 

The areas on which coral limestone is now forming are large. 
The Great Barrier Reef of Australia, which lies off the north- 
eastern coast, is twelve hundred and fifty miles long, and has a 
width of from ten to ninety miles. Most of the islands of the 
tropics are either skirted with coral reefs or are themselves of 
coral formation. 

Conditions of coral growth. Reef-building corals cannot live 
except in clear salt water less, as a rule, than one hundred and 
fifty feet in depth, with a winter temperature not lower than 
68 F. An important condition also is an abundant food sup- 
ply, and this is best secured in the path of the warm oceanic 
currents. 

Coral reefs may be grouped in three classes, fringing reefs, 
barrier reefs, and atolls. 

Fringing reefs. These take their name from the fact that they are 
attached as narrow fringes to the shore. An example is the reef which 
forms a selvage about a mile wide along the northeastern coast of 
Cuba. The outer margin, indicated by the line of white surf, where 
the corals are in vigorous growth, rises from about forty feet of water. 



OFFSHORE AND DEEP-SEA DEPOSITS 191 

Between this and the shore lies a stretch of shoal across which one can 
wade at low water, composed of coral sand with here and there a clump 
of growing coral. 

Barrier reefs. Eeefs separated from the shore by a ship 
channel of quiet water, often several miles in width and some- 
times as much as three hundred feet in depth, are known as 
barrier reefs. The seaward face rises abruptly from water too 
deep for coral growth. Low islands are cast up by the waves 
upon the reef, and inlets give place for the ebb and flow of the 
tides. Along the west coast of the island of New Caledonia 
a barrier reef extends for four hundred miles, and for a length 
of many leagues seldom approaches within eight miles of the 
shore. 

Atolls. These are ring-shaped or irregular coral islands, or 
island-studded reefs, inclosing a central lagoon. The narrow 
zone of land, like the rim of a great bowl sunken to the water's 
edge, rises hardly more than twenty feet at most above the sea, 
and is covered with a forest of trees such as the cocoanut, whose 
seeds can be drifted to it uninjured from long distances. The 
white beach of coral sand leads down to the growing reef, on 
whose outer margin the surf is constantly breaking. The sea 
face of the reef falls off abruptly, often to depths of thousands 
of feet, while the lagoon varies in depth from a few feet to one 
hundred and fifty or two hundred, and exceptionally measures 
as much as three hundred and fifty feet. 

Theories of coral reefs. Fringing reefs require no explanation, 
since the depth of water about them is not greater than that at 
which coral can grow ; but barrier reefs and atolls, which may 
rise from depths too great for coral growth demand a theory of 
their origin. 

Darwin's theory holds that barrier reefs and atolls are formed 
from fringing reefs by subsidence. The rate of sinking cannot 
be greater than that of the upbuilding of the reef, since other- 
wise the corals would be carried below their depth and drowned. 



192 THE ELEMENTS OF GEOLOGY 

The process is illustrated in Figure 161, where v represents a vol- 
canic island in mid ocean undergoing slow depression, and ss the 
sea level before the sinking began, when the island was surrounded 
by a fringing reef. As the island slowly sinks, the reef builds 
up with equal pace. It rears its seaward face more steep than 
the island slope, and thus the intervening space between the sink- 
ing, narrowing land and the outer margin of the reef constantly 
widens. In this intervening space the corals are more or less 
smothered with silt from the outer reef and from the land, and 
are also deprived in large measure of the needful supply of food 




FIG. 161. Diagram illustrating the Subsidence Theory of Coral Reefs 

and oxygen by the vigorous growth of the corals on the outer 
rim. The outer rim thus becomes a barrier reef and the inner 
belt of retarded growth is deepened by subsidence to a ship chan- 
nel, s f s' representing sea level at this time. The final stage, 
where the island has been carried completely beneath the sea 
and overgrown by the contracting reef, whose outer ring now 
forms an atoll, is represented by s ff s". 

In several instances, however, atolls and barrier reefs may 
be explained without subsidence. Thus a barrier reef may be 
formed by the seaward growtli of a fringing reef upon the talus 
of its sea face. In Figure 162 / is a fringing reef whose outer 
wall rises from about one hundred and fifty feet, the lower limit 
of the reef-building species. At the foot of this submarine cliff 
a talus of fallen blocks t accumulates, and as it reaches the zone 



OFFSHORE AND DEEP-SEA DEPOSITS 



193 




FIG. 162. Barrier Reef formed 
without Subsidence 

, zone of coral growth; /, former 
fringing reef ; t, talus; 6, .barrier 
reef 



of coral growth becomes the foundation on which the reef is 

steadily extended seaward. As the reef widens, the polyps of 

the circumference nourish, while those of the inner belt are 

retarded in their growth and at 

last perish. The coral rock of the 

inner belt is now dissolved by sea 

water and scoured out by tidal 

currents until it gives place to a 

gradually deepening ship channel, 

while the outer margin is left as 

a barrier reef. 

In much the same way atolls 
may be built on any shoal which lies within the zone of coral 
growth. Such shoals may be produced when volcanic islands 
are leveled by waves and ocean currents, and when subma- 
rine plateaus, ridges, and peaks are built up by various organic 
agencies, such as molluscous and foraminiferal shell deposits 
(Fig. 163). The reef-building corals, whose eggs are drifted 

widely over the tropic seas 
by ocean currents, colonize 
such submarine foundations 
wherever the conditions are 
favorable for their growth. 
As the reef approaches the 
FIG. 163. Section of Atoll on a Shoal surface the corals of the in- 
which has been built up to near the ner area are smothered by 




Surface by Organic Deposits upon a 
Submarine Volcanic Peak 

v, volcano; /, foraminiferal deposits; m, 



silt and starved, and their 
hard parts are dissolved and 

molluscous 'shell deposits; c, coral reef; SCOUred a way ; while those 

of the circumference, with 

abundant food supply, nourish and build the ring of the atoll. 
Atolls may be produced also by the backward drift of sand from 
either end of a crescentic coral reef or islard, the spits uniting 
in the quiet water of the lee to inclose a lagoon. In the Maldive 



194 THE ELEMENTS OF GEOLOGY 

Archipelago all gradations between crescent-shaped islets and 
complete atoll rings have been observed. 

Barrier and fringing reefs are commonly interrupted off the mouths 
of rivers. Why ? 

In many volcanic islands surrounded with barrier reefs the shores of 
the islands are indented with wedge-shaped bays separated by tapering 
spurs. Which theory do such embayed coasts support ? 

On Funafuti atoll a boring was sunk more than a thousand feet 
in lime carbonate rock. What inference may be drawn as to the origin 
of this atoll ? 

Christmas island, in the Indian ocean, which rises eleven hundred 
feet above sea level, is an old volcanic pile. The summit of the volcano 
is covered with thick beds of limestone made up of f oraminiferal remains. 
Upon this limestone rests a rim of hills of coral rock which represent 
the ring of islets of an old atoll. Give the history of the island. 

Summary. We have seen that the ocean bed is the goal to 
which the waste of the rocks of the land at last arrives. Their 
soluble parts, dissolved by underground waters and carried to 
the sea by rivers, are largely built up by living creatures into 
vast sheets of limestone. The less soluble portions the waste 
brought in by streams and the waste of the shore form the 
muds and sands of continental deltas. All of these sea deposits 
consolidate and harden, and the coherent rocks of the land are 
thus reconstructed on the ocean floor. But the destination is 
not a final one. The stratified rocks of the land are for the 
most part ancient deposits of the sea, which have been lifted 
above sea level; and we may believe that the sediments now 
being laid offshore are the " dust of continents to be," and will 
sometime emerge to form additions to the land. We are now to 
study the movements of the earth's crust which restore the sedi- 
ments of the sea to the light of day, and to whose beneficence 
we owe the habitable lands of the present. 



PART II 

. INTERNAL GEOLOGICAL AGENCIES 

CHAPTEE IX 
MOVEMENTS OF THE EARTH'S CRUST 

The geological agencies which we have so far studied 
weathering, streams, underground waters, glaciers, winds, and 
the ocean all work upon the earth f rorn without, and all are 
set in motion by an energy external to the earth, namely, the 
radiant energy of the sun. All, too, have a common tendency 
to reduce the inequalities of the earth's surface by leveling the 
lands and strewing their waste beneath the sea. 

But despite the unceasing efforts of these external agencies, 
they have not destroyed the continents, which still rear their 
broad plains and great plateaus and mountain ranges above the 
sea. Either, then, the earth is very young and the agents of 
denudation have not yet had time to do their work, or they 
have been opposed successfully by other forces. 

We enter now upon a department of our science which treats 
of forces which work upon the earth from within, and increase 
the inequalities of its surface. It is they which uplift and re- 
create the lands which the agents of denudation are continually 
destroying; it is they which deepen the ocean bed and thus 
withdraw its waters from the shores. At times also these forces 
have aided in the destruction of the lands by gradually lower- 
ing them and bringing in the sea. Under the action of forces 
resident within the earth the crust slowly rises or sinks ; from 

196 



196 THE ELEMENTS OF GEOLOGY 

time to time it has been folded and broken ; while vast quanti- 
ties of molten rock have been pressed up into it from beneath 
and outpoured upon its surface. We shall take up these phe- 
nomena in the following chapters, which treat of upheavals 
and depressions of the crust, foldings and fractures of the crust, 
earthquakes, volcanoes, the interior conditions of the earth, 
mineral veins, and metamorphism. 

OSCILLATIONS OF THE CRUST 

Of the various movements of the crust due to internal agen- 
cies we will consider first those called oscillations, which lift or 
depress large areas so slowly that a long time is needed to pro- 
duce perceptible changes of level, and which leave the strata in 
nearly their original horizontal attitude. These movements are 
most conspicuous along coasts, where they can be referred to 
the datum plane of sea level. Slow and tranquil oscillations are 
recorded along many shores. Some shores are emerging from the 
sea ; some are being submerged by it ; and no part of the land 
seems to have been exempt from such changes in the past. 

In the use of sea level as a datum plane, allowance must be made for 
the fact that this level is neither unchangeable nor everywhere the same. 

Considering the gravitative attraction of large mountain masses, what 
would you infer as to differences in distance from the earth's center of 
sea level at San Francisco and New Orleans? at Calcutta and Ceylon? 

What general and local effects on sea level and shore lines would be 
produced by 

1. The melting of the inland ice of Greenland? 

2. The accumulation of ice sheets, such as those of the glacial epoch, 

over Europe and Xorth America? 

3. A renewed uplift of the Appalachians raising them to Alpine 

height? 

4. The filling of the Gulf of Mexico with river silts ? 

5. A downwarp of the floor of the Gulf of Mexico doubling the 

capacity of the basin? 
0. A more and a less rapid rotation of the earth ? 



MOVEMENTS OF THE EARTH'S CRUST 197 

Evidences of changes of level. Taking the surface of the 
sea as a level of reference, we may accept as proofs of relative 
upheaval whatever is now found in place above sea level and 
could have been formed only at or beneath it, and as proofs of 
relative subsidence whatever is now found beneath the sea and 
could only have been formed above it. 

Thus old strand lines with sea cliffs, wave-cut rock benches, 
and beaches of wave-worn pebbles or sand, are striking proofs 
of recent emergence to the amount of their present height above 
tide. No less conclusive is the presence of sea-laid rocks which 
we may find in the neighboring quarry or outcrop, although it 
may have been long ages since they were lifted from the sea to 
form part of the dry land. 

Among common proofs of subsidence are roads and buildings 
and other works of man, and vegetal growths and deposits, such 
as forest grounds and peat beds, now submerged beneath the sea. 
In the deltas of many large rivers, such as the Po, the Nile, the 
Ganges, and the Mississippi, buried soils prove subsidences of 
hundreds of feet; and in several cases, as in the Mississippi 
delta, the depression seems to be now in progress. 

Other proofs of the same movement are drowned land forms 
which are modeled only in open air. Since rivers cannot cut 
their valleys farther below the baselevel of the sea than the 
depths of their channels, drowned valleys are among the plainest 
proofs of depression. To this class belong Narragansett, Dela- 
ware, Chesapeake, Mobile, and San Francisco bays, and many 
other similar drowned valleys along the coasts of the United 
States. Less conspicuous are the submarine channels which, 
as soundings show, extend from the mouths of a number of 
rivers some distance out to sea. Such is the submerged chan- 
nel which reaches from New York Bay southeast to the edge 
of the continental shelf, and which is supposed to have been 
cut by the Hudson Eiver when this part of the shelf was a 
coastal plain. 



198 THE ELEMENTS OF GEOLOGY 

Warping. In a region undergoing changes of level the rate 
of movement commonly varies in different parts. Portions of 
an area may be rising or sinking while adjacent portions are 
stationary or moving in the opposite direction. In this way a 
land surface becomes warped. Thus, the eastern end of the 
island of Crete is sinking, for ruins of ancient buildings are 
now seen off shore beneath the water ; while the west and south 
coasts are rising, as is proved by old docks which now stand 
twenty-seven feet above sea level. 

Since the close of the glacial epoch the coasts of Newfoundland 
and Labrador have risen hundreds of feet, but the rate of emergence 
v o ^ has not been uniform. The old 

<3 strand line, which stands at 

| ; five hundred and seventy-five 

f % feet above tide at St. John's, 

Newfoundland, declines to two 
hundred and fifty feet near 
sea Level ^ Q northern point of Labrador 
FIG. 164. "NVarped Strand Line from (Fig. 164). 

St. John's, Newfoundland, to Nach- L a ke shores. In the interior 

vak Labrador p .- 

oi continents warpings are reg- 
istered by lakes. Any canting of the basin of a lake causes the water 
to rise on one side and to withdraw upon the other. The old strand line 
is left tilted according to the measure of the canting. 

Thus, the strand line of Lake Bonneville (p. 107) is no longer hori- 
zontal, but in some parts stands three hundred and fifty feet higher than 
in others. The basins of the Karst (p. 47) show old terraces which de- 
cline toward the southwest. One of these basins nearest to the sea, that 
of Scutari, filled with fresh ground water, is depressed so that its bottom 
is one hundred and twenty-four feet below sea level. Compare this 
warping with the character of the Dalmatian coast (p. 170). 

The shores of the Great Lakes show evidences of warping now in 
progress. On the southwest shore of Lake Superior the rising water is 
invading forests and encroaching upon roads. It has converted the 
mouths of rivers into estuaries. Advancing up valleys, it has effaced 
rapids within historic times. On the opposite side of the lake the en- 
tering streams are swift and shallow. 



MOVEMENTS OF THE EARTH'S CRUST 199 

At the western end of Lake Erie are found submerged caves contain- 
ing stalactites, and old meadows and forest grounds are now under 
water. In Sandusky Bay the water is rising at the rate of about two 
feet per century. 

The ancient beaches of the expanded lakes which occupied the basins 
of the Great Lakes at the close of the glacial epoch decline to the south 
and west and show a long-continued rise of the land to the northeast. 

Physiographic effects of oscillations. We have already men- 
tioned several of the most important effects of movements of 
elevation and depression, such as their effects on rivers, the 
mantle of waste (pp. 85, 86), and the forms of coasts (p. 166). 
Movements of elevation including uplifts by folding and 
fracture of the crust to be noticed later are the necessary 
conditions for erosion by whatever agent. They determine the 
various agencies which are to be chiefly concerned in the wear 
of any land, whether streams or glaciers, weathering or the 
wind, and the degree of their efficiency. The lands must 
be uplifted before they can be eroded, and since they must be 
eroded before their waste can be deposited, movements of ele- 
vation are a prerequisite condition for sedimentation also. Sub- 
sidence is a necessary condition for deposits of great thickness, 
such as those of the Great Valley of California and the Indo- 
Gangetic plain (p. 101), the Mississippi delta (p. 109), and the 
still more important formations of the continental delta in 
gradually sinking troughs (p. 183). It is not too much to say 
that the character and thickness of each formation of the strati- 
fied rocks depend primarily on these crustal movements. 

Along the Baltic coast of Sweden, bench marks show that the sea is 
withdrawing from the land at a rate which at the north amounts to 
between three and four feet per century. Towards the south the rate 
decreases. South of Stockholm, until recent years, the sea has gained 
upon the land, and here in several seaboard towns streets by the shore 
are still submerged. The rate of oscillation increases also from the coast 
inland. On the other hand, along the German coast of the Baltic the 
only historic fluctuations of sea level are those which may be accounted 




200 



MOVEMENTS OF THE EARTH'S CRUST 



201 



for by variations due to changes in rainfall. In 1730 Celsius explained 
the changes of level of the Swedish coast as due to a lowering of the 
Baltic instead of to an elevation of the land. Are the facts just stated 
consistent with his theory? 

At the little town of Tadousac where the Saguenay River emp- 
ties into the St. Lawrence there are terraces of old sea beaches, some 
almost as fresh as recent railway fills, the highest standing two hundred 
and thirty feet above the river 
(Fig. 165). Here the Saguenay 
is eight hundred and forty feet 
in depth, and the tide ebbs 
and flows far up its stream. 
Was its channel cut to this 




"- 



FIG. 166. Diagram showing Ruins of 
Temple, North of Naples 



C, ancient sea cliff ; m, marble pillars, 
dotted where bored by mollusks; si, 
present sea level 



depth by the river when the 
land was. at its present height? 
What oscillations are here re- 
corded, and to what amount ? 

A few miles north of Naples, Italy, the ruins of an ancient Roman 
temple lie by the edge of the sea, on a narrow plain which is overlooked 
in the rear by an old sea cliff (Fig. 166). Three marble pillars are still 
standing. For eleven feet above their bases these columns are unin- 
jured, for to this height they were protected by an accumulation of 
volcanic ashes ; but from eleven to nineteen feet they are closely pitted 
with the holes of boring marine mollusks. From these facts trace the 
history of the oscillations of the region. 



FOLDINGS OF THE CRUST 

The oscillations which we have just described leave the strata 
not far from their original horizontal attitude. Figure 167 repre- 
sents a region in which movements of a very different nature 




FIG. 167. Section in a Region of Folded Rocks 



202 



THE ELEMENTS OF GEOLOGY 




have taken place. Here, on either side of the valley v, we 
find outcrops of layers tilted at high angles. Sections along 
the ridge r show that it is composed of layers which slant 
inward from either side. In places the 
outcropping strata stand nearly on edge, 
and on the right of the valley they are 
quite overturned; a shale sh has come 
FIG. 168. Dip and Strike to overlie a limestone Im, although the 
shale is the older rock, whose original position was beneath the 
limestone. 

It is not reasonable to suppose that these rocks were deposited 
in the attitude in which we find them now ; we must believe 
that, like other stratified rocks, they were outspread in nearly 
level sheets upon the ocean floor. Since that time they must 
have been deformed. Layers of solid rock several miles in 
thickness have been crumpled and folded like soft wax in the 
hand, and a vast denudation has worn away the upper portions 
of the folds, in part represented in our section by dotted lines. 

Dip and strike. In 
districts where the 
strata have been dis- 
turbed it is desirable to 
record their attitude. 
This is most easily done 
by taking the angle at 
which the strata are in- 
clined and the compass 
direction in which they 

slant. It is also con- 

FIG. 169. An Anticline, Maryland 
vement to record the 

direction in which the outcrop of the strata trends across the 
country. 

The inclination of a bed of rocks to the horizon is its dip 
(Fig. 168). The amount of the dip is the angle made with a 




MOVEMENTS OF THE EARTH'S CRUST 



203 



horizontal plane. The dip of a horizontal layer is zero, and that 
of a vertical layer is 90. The direction of the dip is taken 
with the compass. Thus a geologist's notebook in describing 
the attitude of outcropping strata contains many such entries 
as these : dip 32 north, or dip 8 south 20 west, meaning in 
the latter case that the amount of the dip is 8 and the direc- 
tion of the dip bears 20 west of south. 




FIG. 170. Folded Strata, Coast of England 
A syncline in the center, with an anticline on either side 

The line of intersection of a layer with the horizontal plane 
is the strike. The strike always runs at right angles to the dip. 

Dip and strike may be illustrated by a book set aslant on a shelf. 
The dip is the acute angle made with the shelf by the side of the 
book, while the strike is represented by a line running along the book's 
upper edge. If the dip is north or south, the strike runs east and west. 

Folded structures. An upfold, in which the strata dip away 
from a line drawn along the crest and called the axis of the 
fold, is known as an anticline (Fig. 169). A downfold, where 
the strata dip from either side toward the axis of the trough, is 



204 



THE ELEMENTS OF GEOLOGY 




called a syncline (Fig. 170). There is sometimes seen a down- 
ward bend in horizontal or gently inclined 'strata, by which they 

descend to a lower level. 
Such a single flexure is a 
monocline (Fig. 171). 

Degrees of folding. Folds 
vary in degree from broad, 
low swells, which can hard- 
ly be detected, to the most 
FIG. 171. A Monocline highly contorted and com- 

plicated structures. In symmetric folds (Figs. 169 and 180) the 
dips of the rocks on each side the axis of the fold are equal. 
In unsymmetrical folds one limb is steeper than the other, as 
in the anticline in Figure 167. In overturned folds (Figs. 167 
and 172) one limb is inclined beyond the perpendicular. Fan 
folds have been so pinched that the original anticlines are left 
broader at the top than at the bottom (Fig. 173). 




FIG. 172. Overturned Fold, Vermont 

In folds where the compression has been great the layers are often 
found thickened at the crest and thinned along the limbs (Fig. 174). 
Where strong rocks such as heavy limestones are folded together with 



MOVEMENTS OF THE EARTH'S CRUST 



205 




weak rocks such as shales, the strong rocks are often bent into great 
simple folds, while the weak rocks are minutely crumpled. 

Systems of folds. As a rule, folds occur in systems. Over 
the Appalachian mountain belt, for example, extending from 
northeastern Pennsylvania to northern Alabama and Georgia, 
the earth's crust has 
been thrown into a 
series of parallel folds 
whose axes run from 
northeast to south- 
west (Fig. 175). In 
Pennsylvania one FlG ' 17a Fan Folds ' the Alps 

may count a score or more of these earth waves, some but from 
ten to twenty miles in length, and some extending as much as 
two hundred miles before they die away. On the eastern part 
of this belt the folds are steeper and more numerous than on 
the western side. 

Cause and conditions of folding. The sections which we have 
studied suggest that rocks are folded by lateral pressure. While 
a single, simple fold might be produced by a heave, a series of 
folds, including overturns, fan folds, and folds thickened on 
then- crests at the expense of their limbs, could only be made 

hi one way, by pressure from the 
side. Experiment has reproduced all 
forms of folds by subjecting to lat- 
eral thrust layers of plastic material 
such as wax. 

Vast as the force must have been 
which could fold the solid rocks of 
the crust as one may crumple the 




FIG. 174. Folds with Layers 
thickened at the Crest 
and thinned along the 
Limbs 



leaves of a magazine in the fingers, it is only under certain con- 
ditions that it could have produced the results which we see. 
Rocks are brittle, and it is only when under a heavy load, and 
by great pressure slowly applied, that they can thus be folded 



206 



THE ELEMENTS OF GEOLOGY 




FIG. 175. Relief Map of the Northern Appalachian Region 
From Brigham's Geographic Influences in American History 

and bent instead of being crushed to pieces. Under these con- 
ditions, experiments prove that not only metals such as steel, 
but also brittle rocks such as marble, can be deformed and 
molded and made to flow like plastic clay. 



MOVEMENTS OF THE EARTH'S CRUST 207 

Zone of flow, zone of flow and fracture, and zone of fracture. 

We may believe that at depths which must be reckoned in tens 
of thousands of feet the load of overlying rocks is so great that 
rocks of all kinds yield by folding to lateral pressure, and flow 
instead of breaking. Indeed, at such profound depths and under 
such inconceivable weight no cavity can form, and any fractures 
would be healed at once by the welding of grain to grain. At 
less depths there exists a zone where soft rocks fold and flow 
under stress, and hard rocks are fractured ; while at and near 
the surface hard and soft rocks alike yield by fracture to strong 
pressure. 

STRUCTURES DEVELOPED IN COMPRESSED KOCKS 

Deformed rocks show the effects of the stresses to which 
they have yielded, not only in the immense folds into which 
they have been thrown but hi their smallest parts as well. 
A hand specimen of slate, or even a particle under the micro- 
scope, may show plications similar in form and origin to the 
foldings which have produced ranges of mountains. A tiny 
flake of mica in the rocks of the Alps may be puckered by the 
same resistless forces which have folded miles of solid rock to 
form that lofty range. 

Slaty cleavage. Eocks which have yielded to pressure often 
split easily in a certain direction across the bedding planes. 
This cleavage is known as slaty cleavage, since it is most per- 
fectly developed in fine-grained, homogeneous rocks, such as 
slates, which cleave to the thin, smooth-surfaced plates with 
which we are familiar in the slates used in roofing and for 
ciphering and blackboards. In coarse-grained rocks, pressure 
develops more distant partings which separate the rocks into 
blocks. 

Slaty cleavage cannot be due to lamination, since it commonly 
crosses bedding planes at an angle, while these planes have 
been often well-nigh or quite obliterated. Examining slate with 



208 THE ELEMENTS OF GEOLOGY 

a microscope, we find that its cleavage is due to the grain of 
the rock. Its particles are flattened and lie with their broad 
faces in parallel planes, along which the rock naturally splits 
more easily than in any other direction. The irregular grains 
of the mud which has been altered to slate have been squeezed 

flat by a pressure ex- 
erted at right angles 
to. the plane of cleav- 
age. Cleavage is 
found only in folded 
rocks, and, as we 

S6e 




Fio.176. Slaty Cleavage 

176, the strike of the 

cleavage runs parallel to the strike of the strata and the axis 
of the folds. The dip of the cleavage is generally steep, hence 
the pressure was nearly horizontal. The pressure which has 
acted at right angles to the cleavage, and to which it is due, is 
the same lateral pressure which has thrown the strata into folds. 

We find additional proof that slates have undergone compression at 
right angles to their cleavage in the fact that any inclusions in them, 
such as nodules and fossils, have been squeezed out of shape and have 
their long diameters lying in the planes of cleavage. 

That pressure is competent to cause cleavage is shown by experi- 
ment. Homogeneous material of fine grain, such as beeswax, when 
subjected to heavy pressure cleaves at right angles to the direction of 
the compressing force. 

Rate of folding. All the facts known with regard to rock 
deformation agree that it is a secular process, taking place so 
slowly that, like the deepening of valleys by erosion, it escapes 
the notice of the inhabitants of the region. It is only under 
stresses slowly applied that rocks bend without breaking. The 
folds of some of the highest mountains have risen so gradually 
that strong, well-intrenched rivers which had the right of way 
across the region were able to hold to their courses, and as 



MOVEMENTS OF THE EARTH'S CRUST 



209 



a circular saw cuts its way through the log which is steadily 
driven against it, so these rivers sawed their gorges through 
the fold as fast as it rose beneath them. Streams which thus 
maintain the course which they had antecedent to a deforma- 
tion of the region are known as antecedent streams. Examples 
of such are the Sutlej and other rivers of India, whose valleys 
trench the outer ranges of the Himalayas and whose earlier 
river deposits have been upturned by the rising ridges. On 
the other hand, mountain crests are usually divides, parting the 
head waters of different drainage systems. In these cases the 
original streams of the region have been broken or destroyed by 
the uplift of the mountain mass across their paths. 

On the whole, which have worked more rapidly, processes of defor- 
mation or of denudation ? 



LAND FORMS DUE TO FOLDING 

As folding goes on so slowly, it is never left to form surface 
features unmodified by the action of other agencies. An anti- 
clinal fold is attacked by 
erosion as soon as it begins 
to rise above the original 
level, and the higher it is 
uplifted, and the stronger 
are its slopes, the faster is 
it worn away. Even while 
rising, a young upfold is 
often thus unroofed, and 
instead of appearing as a 
long, smooth, boat-shaped FlG - m - An Unroofed Anticline 

ridge, it commonly has had opened along the rocks of the axis, 
when these are weak, a valley which is overlooked by the in- 
facing escarpments of the hard layers of the sides of the fold 
(Fig. 177). Under long-continued erosion, anticlines may be 




210 



THE ELEMENTS OF GEOLOGY 



degraded to valleys, while the synclines of the same system 
may be left in relief as ridges (Fig. 167). 

Folded mountains. The vastness of the forces which wrin- 
kle the crust is best realized in the presence of some lofty 
mountain range. All mountains, indeed, are not the result of 
folding. Some, as we shall see, are due to upwarps or to frac- 
tures of the crust ; some are piles of volcanic material ; some 




FIG. 178. Mountain Peaks carved in Folded Strata, Rocky 
Mountains, Montana 

are swellings caused by the intrusion of molten matter beneath 
the surface ; some are the relicts left after the long denudation 
of high plateaus. 

But most of the mountain ranges of the earth, and some of 
the greatest, such as the Alps and the Himalayas, were origi- 
nally mountains of folding. The earth's crust has wrinkled into 
a fold ; or into a series of folds, forming a series of parallel 
ridges and intervening valleys ; or a number of folds have been 
mashed together into a vast upswelling of the crust, in which 
the layers have been so crumpled and twisted, overturned and 



MOVEMENTS OF THE EARTH'S CRUST 211 

crushed, that it is exceedingly difficult to make out the origi- 
nal structure. 

The close and intricate folds seen in great mountain ranges 
were formed, as we have seen, deep below the surface, within the 
zone of folding. Hence they may never have found expression 
in any individual surface features. As the result of these defor- 
mations deep under ground the surface was broadly lifted to 
mountain height, and the crumpled and twisted mountain 




FIG. 179. Section of a Portion of the Alps 

structures are now to be seen only because erosion has swept 
away the heavy cover of surface rocks under whose load they 
were developed. 

When the structure of mountains has been deciphered it is possible 
to estimate roughly the amount of horizontal compression which the 
region has suffered. If the strata of the folds of the Alps were smoothed 
out, they would occupy a belt from four hundred to seven hundred and 
fifty miles wide, while the width to which they have been compressed 
is but one hundred miles. A section across the Appalachian folds in 
Pennsylvania shows a compression to about two thirds the original 
width ; the belt has been shortened thirty-five miles in every hundred. 

Considering the thickness of their strata, the compression which moun- 
tains have undergone accounts fully for their height, with enough to 
spare for all that has been lost by denudation. 

The Appalachian folds involve strata thirty thousand feet in thick- 
ness. Assuming that the folded strata rested on an unyielding founda- 
tion, and that what was lost in width was gained in height, what elevation 
would the range have reached had not denudation worn it as it rose ? 



212 THE ELEMENTS OF GEOLOGY 

The life history of mountains. While the disturbance and 
uplift of mountain masses are due to deformation, their sculp- 
ture into ridges and peaks, valleys and deep ravines, and all 
the forms which meet the eye in mountain scenery, excepting 
in the very youngest ranges, is due solely to erosion. We may 
therefore classify mountains according to the degree to which 
they have been dissected. The Juras are an example of the 
stage of early youth, in which the anticlines still persist as ridges 
and the synclines coincide with the valleys ; this they owe as 
much to the slight height of their uplift as to the recency of its 
date (Fig. 180). 

The Alps were upheaved at various times (p. 399), the last 
uplift being later than the uplift of the Juras, but to so much 
greater height that erosion has already advanced them well on 



FIG. 180. Section of a Portion of the Jura Mountains 

towards maturity. The mountain mass has been cut to the core, 
revealing strange contortions of strata which could never have 
found expression at the surface. Sharp peaks, knife-edged crests, 
deep valleys with ungraded slopes subject to frequent landslides, 
are all features of Alpine scenery typical of a mountain range 
at this stage in its life history. They represent the survival of 
the hardest rocks and the strongest structures, and the destruc- 
tion of the weaker in their long struggle for existence against 
the agents of erosion. Although miles of rock have been re- 
moved from such ranges as the Alps, we need not suppose that 
they ever stood much, if any, higher than at present. All this 
vast denudation may easily have been accomplished while their 
slow upheaval was going on ; in several mountain ranges we 
have evidence that elevation has not yet ceased. 

Under long denudation mountains are subdued to the forms 
characteristic of old age. The lofty peaks and jagged crests of 




213 



214 



THE ELEMENTS OF GEOLOGY 



their earlier life are smoothed down to low domes and rounded 
crests. The southern Appalachians and portions of the Hartz 
Mountains in Germany (Fig. 182) are examples of mountains 
which have reached this stage. 

There are numerous regions of upland and plains in which 
the rocks are found to have the same structure that we have 
seen in folded mountains ; they are tilted, crumpled, and over- 
turned, and have clearly suffered intense compression. We may 




FIG. 182. Subdued Mountains, the Hartz Mountains, Germany 

infer that their folds were once lifted to the height of mountains 
and have since been wasted to low-lying lands. Such a section 
as that of Figure 67 illustrates how ancient mountains may be 
leveled to their roots, and represents the final stage to which 
even the Alps and the Himalayas must sometime arrive. 
Mountains, perhaps of Alpine height, once stood about Lake 
Superior ; a lofty range once extended from New England and 
New Jersey southwestward to Georgia along the Piedmont belt. 
In our study of historic geology we shall see more clearly how 



MOVEMENTS OF THE EARTH'S CRUST 215 

short is the life of mountains as the earth counts time, and how 
great ranges have been lifted, worn away, and again upheaved 
into a new cycle of erosion. 

The sedimentary 'history of folded mountains. We may men- 
tion here some of the conditions which have commonly been 
antecedent to great foldings of the crust. 

1. Mountain ranges are made of belts of enormously and 
exceptionally thick sediments. The strata of the Appalachians 
are thirty thousand feet thick, while the same formations thin 
out to five thousand feet in the Mississippi valley. The folds of 
the Wasatch Mountains involve strata thirty thousand feet thick, 
which thin to two thousand feet in the region of the Plains. 

2. The sedimentary strata of which mountains are made are 
for the most part the shallow-water deposits of continental 
deltas. Mountain ranges have been upfolded along the margins 
of continents. 

3. Shallow-water deposits of the immense thickness found 
in mountain ranges can be laid only in a gradually sinking area. 
A profound subsidence, often to be reckoned in tens of thou- 
sands of feet, precedes the upfolding of a mountain range. 

Thus the history of mountains of folding is as follows : For 
long ages the sea bottom off the coast of a continent slowly 
subsides, and the great trough, as fast as it forms, is filled with 
sediments, which at last come to be many thousands of feet 
thick. The downward movement finally ceases. A slow but 
resistless pressure sets in, and gradually, and with a long series 
of many intermittent movements, the vast mass of accumulated 
sediments is crumpled and uplifted into a mountain range. 

FRACTURES AND DISLOCATIONS OF THE CRUST 

Considering the immense stresses to which the rocks of the 
crust are subjected, it is not surprising to find that they often 
yield by fracture, like brittle bodies, instead of by folding and 



216 THE ELEMENTS OF GEOLOGY 

flowing, like plastic solids. Whether rocks bend or break de- 
pends on the character and condition of the rocks, the load of 
overlying rocks which they bear, and the amount of the force 
and the slowness with which it is applied. * 

Joints. At the surface, where their load is least, we find rocks 
universally broken into blocks of greater or less size by partings 
known as joints. Under this name are included many division 
planes caused by cooling and drying; but it is now generally 
believed that the larger and more regular joints, especially those 




FIG. 183. Joints utilized by a River in widening its Valley, Iowa 

which run parallel to the dip and strike of the strata, are frac- 
tures due to up-and-down movements and foldings and twistings 
of the rocks. 

Joints are used to great advantage in quarrying, and we have 
seen how they are utilized by the weather in breaking up rock 
masses, by rivers in widening their valleys, by the sea in driving 
back its cliffs, by glaciers in plucking their beds, and how they 
are enlarged in soluble rocks to form natural passageways for 
underground waters. The ends of the parted strata match along 




MOVEMENTS OF THE EARTH'S CRUST 217 

both sides of joint planes ; in joints there has been little or no 
displacement of the broken rocks. 

Faults. In Figure 184 the rocks have been both broken and 
dislocated along the plane ff. One side must have been moved 
up or down past the other. Such a dislocation is called a fault. 
The amount of the displacement, as measured by the vertical 
distance between the ends of a parted layer, is the throw (cd). 
The angle (ffv) which the fault plane ^. v 

makes with the vertical is the hade. 
In Figure 184 the right side has gone 
down relatively to the left; the right 
is the side of the downthrow, while 
the left is the side of the upthrow. 
Where the fault plane is not vertical the 
surfaces on the two sides may be dis- 
tinguished as the hanging wall (that on the right of Figure 184) 
and the foot wall (that on the left of the same figure). Faults 
differ in throw from a fraction of an inch to many thousands 
of feet. 

Slicken^ides. If we examine the walls of a fault, we may find further 
evidence of movement in the fact that the surfaces are polished and 
grooved by the enormous friction which they have suffered as they 
have ground one upon the other. These appearances, called slicken- 
sides, have sometimes been mistaken for the results of glacial action. 

Normal faults. Faults are of two kinds, normal faults 
and thrust faults. Normal faults, of which Figure 184 is an 
example, hade to the downthrow; the hanging wall has gone 
down. The total length of the strata has been increased by the 
displacement. It seems that the strata have been stretched and 
broken, and that the blocks have readjusted themselves under 
the action of gravity as they settled. 

Thrust faults. Thrust faults hade to the upthrow; the 
hanging wall has gone up. Clearly such faults, where the strata 
occupy less space than before, are due to lateral thrust. Folds 



218 



THE ELEMENTS OF GEOLOGY 



FIG. 185. A Thrust Fault 



and thrust faults are closely associated. . Under lateral pressure 
strata may fold to a certain point and then tear apart and fault 
along the surface of least resistance. Under immense pressure 
strata also break by shear without folding. Thus, in Figure 185, 
the rigid earth block under lateral thrust has found it easier to 
break along the fault plane than to fold. Where such faults are 

nearly horizontal they are 
distinguished as thrust 
planes. 

In all thrust faults one 
mass has been pushed over 
another, so as to bring the 

underlying and older strata upon younger beds ; and when the 
fault planes are nearly horizontal, and especially when the rocks 
have been broken into many slices which have slidden far one 
upon another, the true succession of strata is extremely hard to 
decipher. 

In the Selkirk Mountains of Canada the basement rocks of the region 
have been driven east for seven miles on a thrust plane, over rocks 
which originally lay thousands of feet above them. , 

Along the western Appalachians, from Virginia to Georgia, the 
mountain folds are broken by more than fifteen parallel thrust planes, 
running from northeast to southwest, along which the older strata have 
been pushed westward over the younger. The longest continuous fault 
has been traced three hundred and seventy-five miles, and the greatest 
horizontal displacement has been estimated at not less than eleven miles. 

Crush breccia. Eocks often do not fault with a clean and 
simple fracture, but along a zone, sometimes several yards in 
width, in which they are broken to fragments. It may occur 
also that strata which as a whole yield to lateral thrust by 
folding include beds of brittle rocks, such as thin-layered lime- 
stones, which are crushed to pieces by the strain. In either 
case the fragments when recemented by percolating waters form 
a rock known as a crush breccia (pronounced bretcha) (Fig. 186). 



MOVEMENTS OF THE EARTH'S CRUST 219 

Breccia is a term applied to any rock formed of cemented 
angular fragments. This rock may be made by the consolida- 
tion of volcanic cinders, of angular waste at the foot of cliffs, or 
of fragments of coral torn by the waves from coral reefs, as well 
as of strata crushed by crustal movements. 

SURFACE FEATURES DUE TO DISLOCATIONS 

Fault scarps. A fault of recent date may be marked at sur- 
face by a scarp, because the face of ,the upthrown block has not 
yet been worn to the 
level of the down- 
throw side. 

After the upthrown 
block has been worn 
down to this level, 
differential erosion 
produces fault scarps 
wherever weak rocks 
and resistant rocks 
are brought in con- 
tact along the fault 
plane ; and the harder 
rocks, whether on the 
upthrow or the down- 
throw side, emerge 

,. e ,., FIG. 186. Breccia 

in a line ot dins. 

Where a fault is so old that no abrupt scarps appear, its general 
course is sometimes marked by the line of division between 
highland and lowland or hill and plain. Great faults have some- 
times brought ancient crystalline rocks in contact with weaker 
and younger sedimentary rocks, and long after erosion has de- 
stroyed all fault scarps the harder crystallines rise in an upland 
of rugged or mountainous country which meets the lowland 
along the line of faulting. 




220 



THE ELEMENTS OF GEOLOGY 



The vast majority of faults give rise to no surface features. 
The faulted region may be old enough to have been baseleveled, 
or the rocks on both sides of the line of dislocation may be 
alike in their resistance to erosion and therefore have been 

worn down to a common slope. 
The fault may be entirely con- 
cealed by the mantle of waste, 
and in such cases it can be in- 
ferred from abrupt changes in 
the character or the strike and 
dip of the strata where they 

may outcrop near it (Fig. 187). 
FIG. 187. A Concealed Fault 

This fault may be inferred from the The plateau trenched by the 
changes in strata in passing along the Qrand Canyon of the Colorado 
strike, as from 6 to a and from c to 6 . 

River exhibits a series of mag- 
nificent fault scarps whose general course is from north to south, mark- 
ing the edges of the great crust blocks into which the country has been 
broken. The highest part of the plateau is a crust block ninety miles 
long arid thirty-five miles in maximum width, which has been hoisted 
to nine thousand three hundred feet above sea level. On the east it 
descends four thousand feet by a monoclinal fold, which passes into a 
fault towards the north. On the west it breaks down by a succession of 





FIG. 188. East-West Section across the Broken Plateau north of the 
Grand Canyon of the Colorado River, Arizona 

terraces faced by fault scarps. The throw of these faults varies from 
seven hundred feet to more than a mile. The escarpments, however, 
are due in a large degree to the erosion of weaker rock on the down- 
throw side. 

The Highlands of Scotland (Fig. 189) meet the Lowlands on the south 
with a bold front of rugged hills along a line of dislocation which runs 



MOVEMENTS OF THE EARTH'S CRUST 



221 



across the country from sea to sea. On the one side are hills of ancient 

crystalline rocks whose crumpled structures prove that they are but the 

roots of once lofty mountains ; on the other lies a lowland of sandstone 

and other stratified rocks formed from the waste of those long-vanished 

mountain ranges. Remnants of sandstone 

occur in places on the north of the great 

fault, and are here seen to rest on the worn 

and fairly even surface of the crystallines. 

We may infer that these ancient mountains 

were reduced along their margins to low FIG. 189. The Fault separat- 

plains, which were slowly lowered beneath 

the sea to receive a cover of sedimentary 




ing the Highlands and the 
Lowlands, Scotland 



rocks. Still later came an uplift and dislocation. On the one side 
erosion has since stripped off the sandstones for the most part, but the 
hard crystalline rocks yet stand in bold relief. On the other side the 
weak sedimentary rocks have been worn down to lowlands. 

Rift valleys. In a broken region undergoing uplift or the 
unequal settling which may follow, a slice inclosed between two 
fissures may sink below the level of the crust blocks on either 
side, thus forming a linear depression known as a rift valley, or 
valley of fracture. 

One of the most striking examples of this rare type of valley is the 
long trough which runs straight from the Lebanon Mountains of Syria 
on the north to the Red Sea on the south, and whose central portion is 

occupied by the Jor- 
dan valley and the 
Dead Sea. The pla- 
teau which it gashes 
has been lifted more 
than three thousand 
feet above sea level, 




a, ancient schists; 6, Carboniferous strata; c, d, and 
e, Cretaceous strata 



FIG. 190. Section from the Mountains of Palestine 

to the Mountains of Moab across the Dead Sea 

sea 

and the bottom of the 
trough reaches a depth 
of two thousand six hundred feet below that level in parts of the Dead 
Sea. South of the Dead Sea the floor of the trough rises somewhat 
above sea level, and in the Gulf of Akabah again sinks below it. This 
uneven floor could be accounted for either by the profound warping of 



222 THE ELEMENTS OF GEOLOGY 

a valley of erosion or by the unequal depression of the floor of a rift 
valley. But that the trough is a true valley of fracture is proved by 
the fact that on either side it is bounded by fault scarps and monoclinal 
folds. The keystone of the arch has subsided. Many geologists believe 
that the Jordan -Akabah trough, the long narrow basin of the Red Sea, 
and the chain of down-faulted valleys which in Africa extends from the 
strait of Bab-el-Mandeb as far south as Lake Nyassa valleys which 
contain more than thirty lakes belong to a single system of dislocation. 
Should you expect the lateral valleys of a rift valley at the time of 
its formation to enter it as hanging valleys or at a common level? 

Block mountains. Dislocations take place on so grand a 
scale that by the upheaval of blocks of the earth's crust or the 
down-faulting of the blocks about one which is relatively sta- 
tionary, mountains known as block mountains are produced. 
A tilted crust block may present a steep slope on the side up- 
heaved and a more gentle descent on the side depressed. 

The Basin ranges. The plateaus of the United States bounded by the 
Rocky Mountains on the east, and on the west by the ranges which 
front the Pacific, have been profoundly fractured arid faulted. The 

system of great fissures 
by which they are broken 
extends north and south, 
and the long, narrow, 
tilted crust blocks inter- 
EIG. 191. Block Mountains, Southern Oregon cepted between the fis- 
sures give rise to the 

numerous north-south ranges of the region. Some of the tilted blocks, 
as those of southern Oregon, are as yet but moderately carved by ero- 
sion, and shallow lakes lie on the waste that has been washed into the 
depressions between them (Fig. 191). We may therefore conclude that 
their displacement is somewhat recent. Others, as those of Nevada, are 
so old that they have been deeply dissected ; their original form has 
been destroyed by erosion, and the intermontane depressions are occupied 
by wide plains of waste. 

Dislocations and river valleys. Before geologists had proved 
that rivers can by their own unaided efforts cut deep canyons, it 




MOVEMENTS OF THE EARTH'S CRUST 



223 



was common to consider any narrow gorge as a gaping fissure 
of the crust. This crude view has long since been set aside. 
A map of the plateaus of northern Arizona shows how inde- 
pendent of the immense faults of the region is the course of the 
Colorado River. In the Alps the tunnels on the Saint Gott- 
hard railway pass six times beneath the gorge of the Reuss, but 
at no point do the rocks show the slightest trace of a fault. 




FIG. 192. Fault crossing Valley in Japan 

Rate of dislocation. So far as human experience goes, the 
earth movements which we have just studied, some of which 
have produced deep-sunk valleys and lofty mountain ranges, 
and faults whose throw is to be measured in thousands of feet, 
are slow and gradual. They are not accomplished by a single 
paroxysmal effort, but by slow creep and a series of slight slips 
continued for vast lengths of time. 

In the Aspen mining district in Colorado faulting is now going on 
at a comparatively rapid rate. Although no sudden slips take place, the 
creep of the rock along certain planes of faulting gradually bends out 



224 THE ELEMENTS OF GEOLOGY 

of shape the square-set timbers in horizontal drifts and has closed 
some vertical shafts by shifting the upper portion across the lower. 
Along one of the faults of this region it is estimated that there has 
been a movement of at least four hundred feet since the Glacial epoch. 
More conspicuous are the instances of active faulting by means of 
sudden slips. In 1891 there occurred along an old fault plane in Japan 
a slip which produced an earth rent traced for fifty miles (Fig. 192). 
The country on one side was depressed in places twenty feet below 
that on the other, and also shifted as much as thirteen feet horizon- 
tally in the direction of the fault line. 

In 1872 a slip occurred for forty miles on the great line of dislocation 
which runs along the eastern base of the Sierra Nevada Mountains. 
In the Owens valley, California, the throw amounted to twenty-five feet 
in places, with a horizontal movement along the fault line of as much as 
eighteen feet. Both this slip and that in Japan just mentioned caused 
severe earthquakes. 

For the sake of clearness we have described oscillations, fold- 
ings, and fractures of the crust as separate processes, each giv- 
ing rise to its own peculiar surface features, but in nature 
earth movements are by no means so simple, they are often 
implicated with one another : folds pass into faults ; in a 
deformed region certain rocks have bent, while others under the 
same strain, but under different conditions of plasticity and 
load, have broken ; folded mountains have been worn to their 
roots, and the peneplains to which they have been denuded 
have been upwarped to mountain height and afterwards dis- 
sected, as in the case of the Alleghany ridges, the southern 
Carpathians, and other ranges, or, as in the case of the Sierra 
Nevada Mountains, have been broken and uplifted as mountains 
of fracture. 

Draw the following diagrams, being careful to show the direction in 
which the faulted blocks have moved, by the position of the two parts 
of some well-defined layer of limestone, sandstone, or shale, which 
occurs on each side of the fault plane, as in Figure 184. 

1. A normal fault with a hade of 15, the original fault scarp 
remaining. 



MOVEMENTS OF THE EARTH'S CRUST 



225 



2. A normal fault with a hade of 50, the original fault scarp worn 
away, showing cliffs caused by harder strata on the downthrow side. 

3. A thrust fault with a hade of 30, showing cliffs due to harder 
strata outcropping on the downthrow. 

4. A thrust fault with a hade of 80, with surface baseleveled. 

5. In a region of normal faults a coal mine is being worked along 
the seam of coal AB (Fig. 193). At B it is found broken 

by a fault /which hades toward A. To find the seam 
again, should you advise tunneling up or down from B1 

6. In a vertical shaft of a coal mine the same bed of 
coal is pierced twice at different levels because of a fault. 
Draw a diagram to show whether the fault is normal or 



L 



'n 

FIG. 193 
i thrust. 




FIG. 194. Ridges to be explained by Faulting 

7. Copy the diagram in Figure 194, showing how the two ridges may 
be accounted for by a single resistant stratum dislocated by a fault. Is 
the fault a strike fault, i.e. one running parallel with the strike of the 
strata, "or a dip fault, one running parallel with the direction of the dip? 




FIG. 195. Earth Block of Tilted Strata, with Included Seam of Coal cc 

8. Draw a diagram of the block in Figure 195 as it would appear if 
dislocated along the plane efg by a normal fault whose throw equals 
one fourth the height of the block. Is the fault a strike or a dip fault? 



226 



THE ELEMENTS OF GEOLOGY 



Draw a second diagram showing the same block after denudation has 
worn it down below the center of the upthrown side. Note that the out- 
crop of the coal seam is now deceptively repeated. This exercise may 
be done in blocks of wood instead of drawings. 





FIG. 196, A and B. Repeated Outcrops of the Same Strata 

9. Draw diagrams showing by dotted lines the conditions both of A 
and of B, Figure 196, after deformation had given the strata their pres- 
ent attitude. 




FIG. 197. A Block Mountain 

10. What is the attitude of the strata of this earth block, Figure 197? 
What has taken place along the plane ftq/"? When did the dislocation 
occur compared with the folding of the strata ? with the erosion of the 
valleys on the right-hand side of the mountain? with the deposition of 
the sediments efgl Do you find any remnants of the original surface 



MOVEMENTS OF THE EARTH'S CRUST 



227 



baf produced by the dislocation? From the left-hand side of the moun- 
tain infer what was the relief of the region before the dislocation. Give 
the complete history recorded in the diagram from the deposition of the 
strata to the present. 




FIG. 198. A Faulted Lava 
Flow aa' 



Scale 1 inch = 1COO feet 

FIG. 199. Measurement of the Thickness 
of Inclined Strata 



11. Which is the older fault, in Figure 198, /or/'? When did the 
lava flow occur? How long a time elapsed between the formation of the 
two faults as measured in the work done in the interval? How long a 
time since the formation of the later fault? 

12. Measure by the scale the thickness be of the coal-bearing strata 
outcropping from a to b in Figure 199. On any convenient scale draw 
a similar section of strata with a dip of 30 outcropping along a hori- 
zontal line normal to the strike one thousand feet in length, and meas- 
ure the thickness of the strata by the scale employed. The thickness 
may also be calculated by trigonometry. 



UNCONFORMITY 

Strata deposited one upon another in an unbroken succession 
are said to be conformable. But the continuous deposition of 
strata is often interrupted by moventents of the earth's crust. 
Old sea floors are lifted to 
form land and are again 
depressed beneath the sea 
to receive a cover of sedi- 
ments only after an inter- 
val during which they FIG. 200. Unconformity between 

Parallel Strata 
were carved by subaenal 

erosion. An erosion surface which thus parts older from youngei 
strata is known as an unconformity, and the strata above it are 



228 



THE ELEMENTS OF GEOLOGY 



said to be unconformable with the rocks below, or to rest uncon- 
formably upon them. An unconformity thus records movements 
of the crust and a consequent break in the deposition of the strata. 
It denotes a period of land erosion of greater or less length, 
which may sometimes be roughly measured by the stage in the 

erosion cycle which the 
land surface had attained 
before its burial. Uncon- 
formable strata may be 
parallel, as in Figure 200, 




FIG. 201. Unconformity between Non- 
parallel Strata 



where the record includes 
the deposition of strata a, 

their emergence, the erosion of the land surface ss, a submer- 

gence and the deposit of the strata I, and lastly, emergence and 

the erosion of the present surface s's'. 

Often the earth movements to which the uplift or depression 

was due involved tilting or folding of the earlier strata, so that 

the strata are now nonparallel as well as unconformable. In 

Figure 201, for example, the record includes deposition, ' uplift, 

and tilting of a\ erosion, depression, the deposit of fr; and finally 

the uplift which has 

brought the rocks to 

open air and permitted 

the dissection by which 

the unconformity is re- 

vealed. 

From this section we 

infer that during early 

Silurian times the area 

was sea, and thick sea 

muds were laid upon it. 

These were later altered 

to hard slates by pressure 
and upfolded into moun- 

tains. During the later 




G 202. Carboniferous Limestones resting 
unconformably on Early Silurian Slates, 
Yorkshire, England 



MOVEMENTS OF THE EARTH'S CRUST 



229 



Silurian and the Devonian the area was land and suffered vast denuda- 
tion. In the Carboniferous period it was lowered beneath the sea and 
received a cover of limestone. 

The age of mountains. It is largely by means of unconformi- 
ties that we read the history of mountain making and other 
deformations and movements of the 
crust. In Figure 203, for example, 
the deformation which upfolded the 
range of mountains took place after 
the deposit of the series of strata a 
of which the mountains are corn- 




FIG. 203. Diagram illustrating 
how the Age of Mountains 
is determined 



posed, and before the deposit of the stratified rocks b, which 
rest unconformably on a and have not shared their uplift. 

Most great mountain ranges, like the Sierra Nevada and the 
Alps, mark lines of weakness along which the earth's crust has 
yielded again and again during the long ages of geological time. 

The strata deposited at 
various times about 
their flanks have been 
infolded by later crump- 
lings with the original 




FIG. 204. Section of Mountain Range showing 
Repeated Uplifts 

a, strata whose folding formed a mountain range ; 
uu, baseleveled surface produced by long de- 
nudation of the mountains; b, tilted strata 
resting unconformably on a; c, horizontal 
strata parted from 6 by the unconformity 
u'u'. The first uplift of the range preceded 
the period of time when 6 was deposited. The 
second uplift, to which the present mountains 
owe their height, was later than this period but 
earlier than the period when strata c were laid 



mountain mass, and 
have been repeatedly 
crushed, inverted, 
faulted, intruded with 
igneous rocks, and de- 
nuded. The structure 
of great mountain 
ranges thus becomes 



exceedingly complex 
and difficult to read. A comparatively simple case of repeated 
uplift is shown in Figure 204. In the section of a portion of 
the Alps shown in Figure 179 a far more complicated history 
may be deciphered. 



230 



THE ELEMENTS OF GEOLOGY 



FIG. 205. Unconformity showing Buried Valleys 

Im, limestone; sh, shale; r, r', and r", river silts filling eroded valleys in the lime- 
stone. The upper surface of the limestone is evidently a land surface devel- 
oped by erosion. The valleys which trench it are narrow and steep-sided; 
hence the land surface had not reached maturity. The sands and muds, now 
hardened to firm rock, which fill these valleys, r, r', and r", contain no relics 
of the sea, but instead the remains of land animals and plants. They are 
river deposits, and we may infer that owing to a subsidence the young rivers 
ceased to degrade their channels and slowly filled their gorges with sands and 
silts. The overlying shale records a further depression which brought the 
land below the level of the sea. A section similar to this is to be seen in the 
coal mines of Bernissant, Belgium, where a gorge twice as deep as that of 
Niagara was discovered, within whose ancient river deposits were found en- 
tombed the skeletons of more than a score of the huge reptiles characteristic 
of the age when the gorge was cut and filled 




FIG. 206. Unconformity showing Buried Mountains, Scotland 

ffn, ancient crystalline rocks ; ss, marine sandstones. The surface bb of the an- 
cient crystalline rocks is mountainous, with peaks rising to a height of as 
much as three thousand feet. It is one of the most ancient land surfaces on 
the planet and is covered unconformably with pre-Cambrian sandstones thou- 
sands of feet in thickness, in which the Torridonian Mountains of Scotland have 
been carved. What has been the history of the region since the mountainous 
surface bb was produced by erosion ? 

Unconformities in the Colorado Canyon, Arizona. How geological his- 
tory may be read in unconformities is further illustrated in Figures 
207 and 208. The dark crystalline rocks a at the bottom of the can- 
yon are among the most ancient known, and are overlain unconformably 
by a mass of tilted coarse marine sandstones 6, whose total thickness 
is not seen in the diagram and measures twelve thousand feet perpen- 
dicularly to the dip. Both a and b rise to a common level rm', and upon 
them rest the horizontal sea-laid strata c, in which the upper portion 
of the canyon has been cut. 



MOVEMENTS OF THE EARTH'S CRUST 



231 



Note that the crystalline rocks a have been crumpled and crushed. 
Comparing their structure with that of folded mountains, what do you 
infer as to their relief after their deformation ? To which surface were 




FIG. 207. Diagram of the Wall of the Colorado Canyon, Arizona, 
showing Unconformities 

they first worn down, mm' or wn? Describe and account for the sur- 
face mm'. How does it differ from the surface of the crystalline rocks 
seen in the Torridonian Mountains (Fig. 206), and why? This sur- 
face mm' is one of the oldest land surfaces of which any vestige remains. 



^_:_~ c 

^rC^j- -n_~- w 




FIG. 208. View of the North Wall of the Grand Canyon of the Colorado 
River, Arizona, showing the Unconformities illustrated in Figure 207 

It is a bit of fossil geography buried from view since the earliest geo- 
logical ages and recently brought to light by the erosion of the canyon. 
How did the surface mm' come to receive its cover of sandstones b ? 
From the thickness and coarseness of these sediments draw inferences 
as to the land mass from which they were derived. Was it rising 



232 THE ELEMENTS OF GEOLOGY 

or subsiding? high or low? Were its streams slow or swift? Was the 
amount of erosion small or great? 

Note the strong dip of these sandstones b. Was the surface mm' 
tilted as now when the sandstones were deposited upon it ? When was 
it tilted ? Draw a diagram showing the attitude of the rocks after this 
tilting occurred, and their height relative to sea level. 

The surface nn' is remarkably even, although diversified by some low 
hills which rise into the bedded rocks of c, and it may be traced for 
long distances up and down the canyon. AVere the layers of b and the 
surface mm' always thus cut short by nn' as now ? What has made the 
surface nn' so even ? How does it come to cross the hard crystalline 
rocks a and the weaker sandstones b at the same impartial level? 
How did the sediments of c come to be laid upon it? Give now the 
entire history recorded in the section, and in addition that involved in 
the production of the platform P, shown in Figure 130, and that of the 
cutting of the canyon. How does the time involved in the cutting 
of the canyon compare with that required for the production of the 
surfaces mm', nn', and P? 



FIG. 209. Unconformity between the Cambrian and Pre-Cambrian Rocks, 

Wisconsin 

z, pre-Carabrian rocks, igneous and metamorphic, greatly deformed ; a', zone 
of decomposed pre-Carabrian rocks and residual clays on which rest the 
Cambrian sandstones b. What unconformity do you find here? What two 
peneplains do you discover? Which is the older? Which was the more com- 
plete? To what stage has the present erosion cycle advanced? Suggest a 
reason why the valleys in the Cambrian are wider than those in the pre- 
Cambrian. When did the decay of the pre-Cambrian rocks of zone a' take 
place, and under what circumstances? Give the entire history recorded in 
this section, stating the successive cycles of erosion in their order and the 
causes which brought each cycle to a close 



CHAPTER X 
EARTHQUAKES 

Any sudden movement of the rocks of the crust, as when 
they tear apart when a fissure is formed or extended, or slip 
from tune to time along a growing fault, produces a jar called 
an earthquake, which spreads in all directions from the place 
of disturbance. 

The Charleston earthquake. On the evening of August 31, 1886, the 
city of Charleston, S. C., was shaken by one of the greatest earthquakes 
which has occurred in the United States. A slight tremor which 
rattled the windows was followed a few seconds later by a roar, as of 
subterranean thunder, as the main shock passed beneath the city. 
Houses swayed to and fro, and their heaving floors overturned furniture 
and threw persons off their feet as, dizzy and nauseated, they rushed 
to the doors for safety. In sixty seconds a number of houses were com- 
pletely wrecked, fourteen thousand chimneys were toppled over, and in 
all the city scarcely a building was left without serious injury. In the 
vicinity of Charleston railways were twisted and trains derailed. Fis- 
sures opened in the loose superficial deposits, and in places spouted 
water mingled with sand from shallow underlying aquifers. 

The point of origin, or focus, of the earthquake was inferred from 
subsequent investigations to be a rent in the rocks about twelve miles 
beneath the surface. From the center of greatest disturbance, which 
lay above the focus, a few miles northwest of the city, the surface 
shock traveled outward in every direction, with decreasing effects, at the 
rate of nearly two hundred miles per minute. It was felt from Boston 
to Cuba, and from eastern Iowa to the Bermudas, over a circular area 
whose diameter was a thousand miles. 

An earthquake is transmitted from the focus through the 
elastic rocks of the crust, as a wave, or series of waves, of com- 
pression and rarefaction, much as a sound wave is transmitted 



234 



THE ELEMENTS OF GEOLOGY 



through the elastic medium of the air. Each earth particle 
vibrates with exceeding swiftness, but over a very short path. 
The swing of a particle in firm rock seldom exceeds one tenth 
of an inch in ordinary earthquakes, and when it reaches one 
half an inch and an inch, the movement becomes dangerous 
and destructive. 

The velocity of earthquake waves, like that of all elastic 
waves, varies with the temperature and elasticity of the medium. 
In the deep, hot, elastic rocks they speed faster than in the 




FIG. 210. Block of the Earth's Crust shaken by an Earthquake 

x, focus; a, b,c,d, successive spheroidal waves in the crust; a', &', c', d', succes- 
sive surface waves produced by the outcropping of a, b, c, and d 

cold and broken rocks near the surface. The deeper the point 
of origin and the more violent the initial shock, the faster and 
farther do the vibrations run. 

Great earthquakes, caused by some sudden displacement or 
some violent rending of the rocks, shake the entire planet. 
Their waves run through the body of the earth at the rate of 
about three hundred and fifty miles a minute, and more slowly 
round its circumference, registering their arrival at opposite 
sides of the globe on the exceedingly delicate instruments of 
modern earthquake observatories. 

Geological effects. Even great earthquakes seldom produce 
geological effects of much importance. Landslides may be 
shaken down from the sides of mountains and hills, and 
cracks may be opened in the surface deposits of plains ; but 



EARTHQUAKES 235 

the transient shiver, which may overturn cities and destroy 
thousands of human lives, runs through the crust and leaves 
it much the same as before. 

Earthquakes attending great displacements. Great earth- 
quakes frequently attend the displacement of large masses of 
the rocks of the crust. In 1822 the coast of Chile was sud- 
denly raised three or four feet, and the rise was five or six feet 
a mile inland. In 1835 the same region was again upheaved 
from two to ten feet. In each instance a destructive earthquake 
was felt for one thousand miles along the coast. 

The great California earthquake of 1906. A sudden dislocation occurred 
in 1906 along an ancient fault plane which extends for 300 miles through 
western California. The vertical displacement did not exceed four 
feet, while the horizontal shifting reached a maximum of twenty feet. 
Fences, rows of trees, and roads which crossed the fault w r ere broken 
and offset. The latitude and longitude of all points over thousands of 
square miles were changed. On each side of the fault the earth blocks 
moved in opposite directions, the block on the east moving southward and 
that on the west moving northward and to twice the distance. East and 
west of the fault the movements lessened with increasing distance from it. 

This sudden slip set up an earthquake lasting sixty-five seconds, 
followed by minor shocks recurring for many days. In places the jar 
shook down the waste on steep hillsides, snapped off or uprooted trees, 
and rocked houses from their foundations or threw down their walls or 
chimneys. The water mains of San Francisco were broken, and the 
city was thus left defenseless against a conflagration which destroyed 
1500,000,000 worth of property. The destructive effects varied with 
the nature of the ground. Buildings on firm rock suffered least, while 
those on deep alluvium were severely shaken by the undulations, like 
water waves, into which the loose material was thrown. Well-braced 
steel structures, even of the largest size, were earthquake proof, and 
buildings of other materials, when honestly built and intelligently 
designed to withstand earthquake shocks, usually suffered little injury. 
The length of the intervals between severe earthquakes in western 
California shows that a great dislocation so relieves the stresses of the 
adjacent earth blocks that scores of years may elapse before the stresses 
again accumulate and cause another dislocation. 



236 THE ELEMENTS OF GEOLOGY 

Perhaps the most violent earthquake which ever visited the United 
States attended the depression, in 1812, of a region seventy-five miles 
long and thirty miles wide, near New Madrid, Mo. Much of the area 
was converted into swamps and some into shallow lakes, while a region 
twenty miles in diameter was bulged up athwart the channel of the 
Mississippi. Slight quakes are still felt in this region from time to 
time, showing that the strains to which the dislocation was due have 
not yet been fully relieved. 

Earthquakes originating beneath the sea. Many earthquakes 
originate beneath the sea, and in a number of examples they 
seem to have been accompanied, as soundings indicate, -by local 
subsidences of the ocean bottom. There have been instances 
where the displacement has been sufficient to set the entire 
Pacific Ocean pulsating for many hours. In mid ocean the wave 
thus produced has a height of only a few feet, while it may be 
two hundred miles in width. On shores near the point of ori- 
gin destructive waves two or three score feet in height roll in, 
and on coasts thousands of miles distant the expiring undula- 
tions may be still able to record themselves on tidal gauges. 

Distribution of earthquakes. Every half hour some consid- 
erable area of the earth's surface is sensibly shaken by an earth- 
quake, but earthquakes are by no means uniformly distributed 
over the globe. As we might infer from what we know as to 
their causes, earthquakes are most frequent in regions now 
.undergoing deformation. Such are young rising mountain ranges, 
fault lines where readjustments recur from time to time, and 
the slopes of suboceanic depressions whose steepness suggests 
that subsidence may there be in progress. 

Earthquakes, often of extreme severity, frequently visit the lofty and 
young ranges of the Andes, while they are little known in the subdued 
old mountains of Brazil. The Highlands of Scotland are crossed by a 
deep and singularly straight depression called the Great Glen, which 
has been excavated along a very ancient line of dislocation. The earth- 
quakes which occur from time to time in this region^ such as the Inver- 
ness earthquake in 1891, are referred to slight slips along this fault plane. 



EARTHQUAKES 237 

In Japan, earthquakes are very frequent. More than a thousand 
are recorded every year, and twenty-nine world-shaking earthquakes 
occurred in the three years ending with 1901. They originate, for the 
most part, well down on the eastern flank of the earth fold whose sum- 
mit is the mountainous crest of the islands, and which plunges steeply 
beneath the sea to the abyss of the Tuscarora Deep. 

Minor causes of earthquakes. Since any concussion with- 
in the crust sets up an earth jar, there are several minor causes 
of earthquakes, such as volcanic explosions and even the col- 
lapse of the roofs of caves. The earthquakes which attend the 
eruption of volcanoes are local, even in the case of the most 
violent volcanic paroxysms known. When the top of a volcano 
has been blown to fragments, the accompanying earth shock has 
sometimes not been felt more than twenty-five miles away. 

Depth of focus. The focus of the Charleston earthquake, 
estimated at about twelve miles below the surface, was excep- 
tionally deep. Volcanic earthquakes are particularly shallow, 
and probably no earthquakes known have started at a greater 
depth than fifteen or twenty miles. This distance is so slight 
compared with the earth's radius that we may say that earth- 
quakes are but skin-deep. 

Should you expect the velocity of an earthquake to be greater in a 
peneplain or in a river delta V 

After an earthquake, piles on which buildings rested were found 
driven into the ground, and chimneys crushed at base. From what 
direction did the shock come ? 

Chimneys standing on the south walls of houses toppled over on the 
roof. Should you infer that the shock in this case came from the north 
or south ? 

How should you expect a shock from the east to affect pictures hang- 
ing on the east and the west walls of a room ? how the pictures hanging 
on the north and the south walls ? 

In parts of the country, as in southwestern Wisconsin, slender 
erosion pillars, or " monuments," are common. What inference could 
you draw as to the occurrence in such regions of severe earthquakes in 
the recent past ? 



CHAPTER XI 
VOLCANOES 

Connected with movements of the earth's crust which take 

place so slowly that they can be inferred only from their effects 

is one of the most rapid and impressive of all geological processes, 

the extrusion of molten rock from beneath the surface of the 

earth, giving rise to all the various phenomena of volcanoes. 

In a volcano, molten rock from a region deep below, which 
we may call its reservoir, ascends through a pipe or fissure to 
the surface. The materials erupted may be spread over vast 
areas, or, as is commonly the case, may accumulate about the 
opening, forming a conical pile known as the volcanic cone. It 
is to this cone that popular usage refers the word volcano ; but 
the cone is simply a conspicuous part of the volcanic mechanism 
whose still more important parts, the reservoir and the pipe, are 
hidden from view. 

Volcanic eruptions are of two types, effusive eruptions, 
in which molten rock wells up from below and flows forth in 
streams of lava (a comprehensive term applied to all kinds of 
rock emitted from volcanoes in a molten state), and explosive 
eruptions, in which the rock is blown out in fragments great 
and small by the expansive force of steam. 

ERUPTIONS OF THE EFFUSIVE TYPE 

The Hawaiian volcanoes. The Hawaiian Islands are all vol- 
canic in origin, and have a linear arrangement characteristic of 
many volcanic groups in all parts of the world. They are strung 
along a northwest-southeast line, their volcanoes standing in 

238 



VOLCANOES 



239 



two parallel rows as if reared along two adjacent lines of frac- 
ture or folding. In the northwestern islands the volcanoes 
have long been extinct and are worn low by erosion. In the 
southeastern island, 
Hawaii, three volca- 
noes are still active and 
in process of building. 
Of these Mauna Loa, 
the monarch of vol- 
canoes, with a girth of 
two hundred miles and 
a height of nearly four- 
teen thousand feet above 
sea level, is a lava dome 
the slope of whose sides does not average more than five 
degrees. On the summit is an elliptical basin ten miles in cir- 
cumference and several hundred feet deep. Concentric cracks 
surround the rim, and from time to time the basin is enlarged 
as great slices are detached from the vertical walls and engulfed. 




FIG. 211. Mauna Loa 




FIG. 212. Caldera of Kilauea 

Such a volcanic basin, formed by the insinking of the top of 
the cone, is called a caldera. 

On the flanks of Mauna Loa, four thousand feet above sea level, lies 
the caldera of Kilauea, an independent volcano whose dome has been 
joined to the larger mountain by the gradual growth of the two. In 



240 THE ELEMENTS OF GEOLOGY 

each caldera the floor, which to the eye is a plain of black lava, is the 
congealed surface of a column of molten rock. At times of an eruption 
lakes of boiling lava appear which may be compared to air holes in a 
frozen river. Great waves surge up, lifting tons of the fiery liquid a 
score of feet in air, to fall back with a mighty plunge and roar, and 
occasionally the lava rises several hundred feet in fountains of dazzling 
brightness. The lava lakes may flood the floor of the basin, but in 




FIG. 213. Portion of the Caldera of Kilauea after a Collapse 
following an Eruption 

historic times have never been known to fill it and overflow the rim. 
Instead, the heavy column of lava breaks way through the sides of the 
mountain and discharges in streams which flow down the mountain 
slopes for a distance sometimes of as much as thirty-five miles. With 
the drawing off of the lava the column in the duct of the volcano 
lowers, and the floor of the caldera wholly or in part subsides. A black 
and steaming abyss marks the place of the lava lakes (Fig. 213). After 
a time the lava rises in the duct, the floor is floated higher, and the boil- 
ing lakes reappear. 



VOLCANOES 241 

The eruptions of the Hawaiian volcanoes are thus of the 
effusive type. The column of lava rises, breaks through the 
side of the mountain, and discharges in lava streams. There 
are no explosions, and usually no earthquakes, or very slight 
ones, accompany the eruptions. The lava in the calderas boils 
because of escaping steam, but the vapor emitted is compara- 
tively little, and seldom hangs above the summits in heavy 
clouds. We see here in its simplest form the most impressive 
and important fact in all volcanic action, molten rock has been 
driven upward to the surface from some deep-lying source. 

Lava flows. As lava issues from the side of a volcano or 
overflows from the summit, it flows away in a glowing stream 




FIG. 214. Pahoehoe Lava, Hawaii 

resembling molten iron drawn white-hot from an iron furnace. 
The surface of the stream soon cools and blackens, and the 
hard crust of nonconducting rock may grow thick and firm 
enough to form a tunnel, within which the fluid lava may flow 
far before it loses its heat to any marked degree. Such tun- 
nels may at last be left as caves by the draining away of the 
lava, and are sometimes several miles in length. 

Pahoehoe and aa. When the crust of highly fluid lava remains 
unbroken after its first freezing, it presents a smooth, hummocky, and 
ropy surface known by the Hawaiian term pdhoehoe (Fig. 214). On the 



242 THE ELEMENTS OF GEOLOGY 

other hand, the crust of a viscid flow may be broken and splintered as it 
is dragged along by the slowly moving mass beneath. The stream then 
appears as a field of stones clanking and grinding on, with here and 
there from some chink a dull red glow or a wisp of steam. It sets to a 
surface called aa, of broken, sharp-edged blocks, which is often both 
difficult and dangerous to traverse (Fig. 215). 




FIG. 215. Lava Flow of the Aa Type ; Cinder Cones in the 
Distance, Arizona 

Fissure eruptions. Some of the largest and most important 
outflows of lava have not been connected with volcanic cones, 
but have been discharged from fissures, flooding the country 
far and wide with molten rock. Sheet after sheet of molten 
rock has been successively outpoured, and there have been 
built up, layer upon layer, plateaus of lava thousands of feet 
in thickness and many thousands of square miles in area. 

Iceland. This island plateau has been rent from time to time by 
fissures from which floods of lava have outpoured. In some instances 
the lava discharges along the whole length of the fissure, but more 
often only at certain points upon it. The Laki fissure, twenty miles 
long, was in eruption in 1783 for seven months. The inundation of 



VOLCANOES 



243 



fluid rock which poured from it is the largest of historic record, reach- 
ing a distance of forty-seven miles and covering two hundred and 
twenty square miles to an average depth of a hundred feet. At the 
present time the fissure is traced by a line of several hundred insignifi- 
cant mounds of f rag- 
mental materials 
which mark where 
the lava issued 
(Fig. 216). 

The distance to 
which the fissure 
eruptions of Iceland 
flow on slopes ex- 
tremely gentle is 
noteworthy. One 
such stream is ninety 
miles in length, and 
another seventy 
miles long has a 
slope of little more 
than one half a de- 




FIG. 216. Small Cinder Cones marking an Eruptive 
Fissure, Iceland 



gree. 

Where 1 ava is 

emitted at one point and flows to a less distance there is gradually built 
up a dome of the shape of an inverted saucer with an immense base but 
comparatively low. Many lava domes have been discovered in Iceland, 
although from their exceedingly gentle slopes, often but two or three 

degrees, they long escaped the 
notice of explorers. 

The entire plateau of Ice- 
land, a region as large as Ohio, 
is composed of volcanic prod- 




FIG. 217. Diagram illustrating the Struc- 
ture of a Lava Plateau such as Iceland 



ucts, for the most part of 
successive sheets of lava whose 
total thickness falls little short 
of two miles. The lava sheets 



If, lava flows ; d, dikes 

exposed to view were outpoured in open air and not beneath the sea ; 
for peat bogs and old forest grounds are interbedded with them, and 
the fossil plants of these vegetable deposits prove that the plateau has 



244 THE ELEMENTS OF GEOLOGY 

long been building and is very ancient. On the steep sea cliffs of the 
island, where its structure is exhibited, the sheets of lava are seen to be 
cut with many dikes, fissures which have been filled by molten rock, 
and there is little doubt that it was through these fissures that the 
lava outwelled in successive flows which spread far and wide over the 
country and gradually reared the enormous pile of the plateau. 



ERUPTIONS OF THE EXPLOSIVE TYPE 

In the majority of volcanoes the lava which rises in the pipe 
is at least in part blown into fragments with violent explosions 
and shot into the air together with vast quantities of water 
vapor and various gases. The finer particles into which the 
lava is exploded are called volcanic dust or volcanic ashes, and 
are often carried long distances by the wind before they settle 
to the earth. The coarser fragments fall about the vent and 
there accumulate in a steep, conical, volcanic mountain. As suc- 
cessive explosions keep open the throat of the pipe, there remains 
on the summit a cup-shaped depression called the crater. 

Stromboli. To study the nature of these explosions we may visit 
Stromboli, a low volcano built chiefly of fragmental materials, which 
rises from the sea off the north coast of Sicily and is in constant 
though moderate action. 

Over the summit hangs a cloud of vapor which strikingly resembles 
the column of smoke puffed from the smokestack of a locomotive, in 
that it consists of globular masses, each the product of a distinct 
explosion. At night the cloud of vapor is lighted with a red glow at 
intervals of a few minutes, like the glow on the trail of smoke behind 
the locomotive when from time to time the fire box is opened. Because 
of this intermittent light flashing thousands of feet above the sea, 
Stromboli has been given the name of the Lighthouse of the Mediter- 
ranean. 

Looking down into the crater of the volcano, one sees a viscid lava 
slowly seething. The agitation gradually increases. A great bubble 
forms. It bursts with an explosion which causes the walls of the 
crater to quiver with a miniature earthquake, and an outrush of steam 



VOLCANOES 245 

carries the fragments of the bubble aloft for a thousand feet to fall 
into the crater or on the mountain side about it. With the explosion 
the cooled and darkened crust of the lava is removed, and the light 
of the incandescent liquid beneath is reflected from the cloud of vapor 
which overhangs the cone. 

At Stromboli we learn the lesson that; the explosive force in 
volcanoes is that of steam. The lava in the pipe is permeated 
with it much as is a thick boiling porridge. The steam in boil- 
ing porridge is unable to escape freely and gathers into bubbles 
which in breaking spurt out drops of the pasty substance; in 
the same way the explosion of great bubbles of steam in the 
viscid lava shoots clots and fragments of it into the air. 

Krakatoa. The most violent eruption of history, that of Krakatoa, a 
small volcanic island in the strait between Sumatra and Java, occurred 
in the last week of August, 1883. Continuous explosions shot a col- 
umn of steam and ashes seventeen miles in air. A black cloud, beneath 
which was midnight darkness and from which fell a rain of ashes 
and stones, overspread the surrounding region to a distance of one 
hundred and fifty miles. Launched on the currents of the upper air, 
the dust was swiftly carried westward to long distances. Three days 
after the eruption it fell on the deck of a ship sixteen hundred miles 
away, and in thirteen days the finest impalpable powder from the vol- 
cano had floated round the globe. For many months the dust hung over 
Europe and America as a faint lofty haze illuminated at sunrise and 
sunset with brilliant crimson. In countries nearer the eruption, as in 
India and Africa, the haze for some time was so thick that it colored 
sun and moon with blue, green, and copper-red tints and encircled 
them with coronas. 

At a distance of even a thousand miles the detonations of the 
eruption sounded like the booming of heavy guns a few miles away. 
In one direction they were audible for a distance as great as that from 
San Francisco to Cleveland. The entire atmosphere was thrown into 
undulations under which all barometers rose and fell as the air waves 
thrice encircled the earth. The shock of the explosions raised sea 
waves which swept round the adjacent shores at a height of more than 
fifty feet, and which were perceptible halfway around the globe. 



246 THE ELEMENTS OF GEOLOGY 

At the close of the eruption it was found that half the mountain had 
been blown away, and that where the central part of the island had 
been the sea was a thousand feet deep. 

Martinique and St. Vincent. In 1902 two dormant volcanoes of the 
West Indies, Mt. Pele"e in Martinique and Soufriere in St. Vincent, 
broke into eruption simultaneously. No lava was emitted, but there 
were blown into the air great quantities of ashes, which mantled the 




FIG. 218. Ruins of St. Pierre, Martinique ; Mt. Pele"e in the Distance 

adjacent parts of the islands with a pall as of gray snow. In early stages 
of the eruption lakes which occupied old craters were discharged and 
swept down the ash-covered mountain valleys in torrents of boiling mud. 
On several occasions there was shot from the crater of each volcano 
a thick and heavy cloud of incandescent ashes and steam, which rushed 
down the mountain side like an avalanche, red with glowing stones and 
scintillating with lightning flashes. Forests and buildings in its path 
were leveled as by a tornado, wood was charred and set on fire by the 
incandescent fragments, all vegetation was destroyed, and to breathe the 



VOLCANOES 



247 



steam and hot, suffocating dust of the cloud was death to every living 
creature. On the morning of the 8th of May, 1902, the first of these 
peculiar avalanches from Mt. Pele"e fell on the city of St. Pierre and 
instantly destroyed the lives of its thirty thousand inhabitants. 




FIG. 219. An Eruption of Vesuvius, 1872 

The huge column of dust and steam rises to a height of about four miles 
above the sea. Drifting down the wind, the vapor condenses into copious 
rains. Such often produce destructive torrents of mud as they sweep down 
the ash-covered mountain side, and during the historic eruption of Vesu- 
vius in A.D. 79 the city of Herculaneum was thus buried. Lava flows are 
marked by the overhanging clouds of aqueous vapor condensed from the 
steam which the molten rock gives off. 



The eruptions of many volcanoes partake of both the effusive 
and the explosive types : the molten rock in the pipe is in part 



248 THE ELEMENTS OF GEOLOGY 

blown into the air with explosions of steam, and in part is dis- 
charged in streams of lava over the lip of the crater and from 
fissures in the sides of the cone. Such are the eruptions of 
Vesuvius, one of which is illustrated in Figure 219. 

Submarine eruptions. The many volcanic islands of the ocean 
and the coral islands resting on submerged volcanic peaks prove 
that eruptions have often taken place upon the ocean floor and 
have there built up enormous piles of volcanic fragments and 
lava. The Hawaiian volcanoes rise from a depth of eighteen thou- 
sand feet of water and lift their heads to about thirty thousand 
feet above the ocean bed. Christmas Island (see p. 194), built 
wholly beneath the ocean, is a coral-capped volcanic peak, whose 
total height, as measured from the bottom of the sea, is more 
than fifteen thousand feet. Deep-sea soundings have revealed 
the presence of numerous peaks which fail to reach sea level 
and which no doubt are submarine volcanoes. A number of vol- 
canoes on the land were submarine in their early stages, as, for 
example, the vast pile of Etna, the celebrated Sicilian volcano, 
which rests on stratified volcanic fragments containing marine 
shells now uplifted from the sea. 

Submarine outflows of lava and deposits of volcanic frag- 
ments become covered with sediments during the long intervals 
between eruptions. Such volcanic deposits are said to be con- 
temporaneous, because they are formed during the same period 
as the strata among which they are imbedded. Contempora- 
neous lava sheets may be expected to bake the surface of the 
stratum on which they rest, while the sediments deposited upon 
them are unaltered by their heat. They are among the most 
permanent records of volcanic action, far outlasting the greatest 
volcanic mountains built in open air. 

From upraised submarine volcanoes, such as Christmas Island, 
it is learned that lava flows which are poured out upon the 
bottom of the sea do not differ materially either in composition 
or texture from those of the land. 



VOLCANOES 



249 



VOLCANIC PRODUCTS 

Vast amounts of steam are, as we have seen, emitted from vol- 
canoes, and comparatively small quantities of other vapors, such 
as various acid and sulphurous gases. The rocks erupted from 
volcanoes differ widely in chemical composition and in texture. 

Acidic and basic lavas. Two classes of volcanic rocks may 
be distinguished, those containing a large proportion of silica 




FIG. 220. Cellular Lava 

(silicic acid, Si0 2 ) and therefore called acidic, and those contain- 
ing less silica and a larger proportion of the bases (lime, magnesia, 
soda, etc.) and therefore called basic. The acidic lavas, of which 
rhyolite and trachyte are examples, are comparatively light in 
color and weight, and are difficult to melt. The basic lavas, of 
which basalt is a type, are dark and heavy and melt at a lower 
temperature. 



260 



THE ELEMENTS OF GEOLOGY 



Scoria and pumice. The texture of volcanic rocks depends in 
part 011 the degree to which they were distended by the steam 
which permeated them when in a molten state. They harden 
into compact rock where the steam cannot expand. Where the 
steam is released from pressure, as on the surface of a lava 
stream, it forms hubbies (steam blebs) of various sizes, which 
give the hardened rock a cellular structure (Fig. 220). In this 




FIG. 221. Amygdules in Lava 

way are formed the rough slags and clinkers called scoria, which 
are found on the surface of flows and which are also thrown 
out as clots of lava in explosive eruptions. 

On the surface of the seething lava in the throat of the vol- 
cano there gathers a rock foam, which, when hurled into the 
air, is cooled and falls as pumice, a spongy gray rock so light 
that it floats on water. 

Amygdules. The steam blebs of lava flows are often drawn 
out from a spherical to an elliptical form resembling that of an 



VOLCANOES 



251 




almond, and after the rock has cooled these cavities are grad- 

ually filled with minerals deposited from solution by under- 

ground water. From their shape such casts are called amygdules 

(Greek, amygdalon, 

an almond). Amyg- 

dules are com- 

monly composed of 

silica. Lavas con- 

tain both silica and 

the alkalies, potash 

and soda, and after 

dissolving the alka- 

lies, percolating 

water is able to take FlG - 222 ' Polished Section of an Agate 

silica also into solution. Most agates are banded amygdules hi 

which the silica has been laid in varicolored, concentric layers 

(Fig. 222). 

Glassy and stony lavas. Volcanic rocks differ in texture 

according also to the rate at which they have solidified. When 

rapidly cooled, as on the surface of 
a lava flow, molten rock chills to a 
glass, because the minerals of which 
it is composed have not had time to 
separate themselves from the fused 
mixture and form crystals. Under 
slow cooling, as in the interior of 
the flow, it becomes a stony mass 
composed of crystals set in a glassy 
paste. In thin slices of volcanic 
lasg Qne may see un de r the micro- 
... , , 

SGO P e the beginnings of crystal 

growth in filaments and needles 
and feathery forms, which are the rudiments of the crystals of 
various minerals. 




FIG. 223. Microsection show- 
ing the Beginnings of Crys- 
tal Growth in Glassy Lava 



252 



THE ELEMENTS OF GEOLOGY 



Spherulites, which also mark the first changes of glassy lavas toward 
a stony condition, are little balls within the rock, varying from micro- 
scopic size to several inches in diameter, 
and made up of radiating fibers. 

Perlitic structure, common among 
glassy lavas, consists of microscopic 
curving and interlacing cracks, due to 
contraction. 




FIG. 224. Perlitic Structure 
and Spherulites, a, a 



Flow lines are exhibited by vol- 
canic rocks both to the naked eye 
and under the microscope. Steam 
blebs, together with crystals and 

their embryonic forms, are left arranged in lines and streaks 

by the currents of the flowing lava as it stiffened into rock. 

Porphyritic structure. Rocks whose ground mass has scat- 
tered through it large conspicuous crystals (Fig. 226) are said 

to be porpliyritic, 

and it is especially 

among volcanic 

rocks that this 

structure occurs. 

The ground mass of 

porphyries either 

may be glassy or 

may consist in part 

of a felt of minute 

crystals ; in either 

case it represents 

the consolidation of 

the rock after its 

outpouring upon 

the surface. On the 

other hand, the large crystals of porphyry have slowly formed 

deep below the ground at an earlier date. 




VOLCANOES 253 

Columnar structure. Just as wet starch contracts on drying 
to prismatic forms, so lava often contracts on cooling to a mass 
of close-set, prismatic, and commonly six-sided columns, which 
stand at right angles to the cooling surface. The upper portion 
of a flow, on rapid cooling from the surface exposed to the air, 




FIG. 226. Porphyritic Structure 

may contract to a confused mass of small and irregular prisms ; 
while the remainder forms large and beautifully regular col- 
umns, which have grown upward by slow cooling from beneath 
(Fig. 227). 

FRAGMENTAL MATERIALS 

Rocks weighing many tons are often thrown from a volcano 
at the beginning of an outburst by the breaking up of the solid- 
ified floor of the crater ; and during the progress of an eruption 
large blocks may be torn from the throat of the volcano by the 
outrush of steam. But the most important f ragmental materials 
are those derived from the lava itself. As lava rises in the pipe, 
the steam which permeates it is released from pressure and 




254 



VOLCANOES 255 

explodes, hurling the lava into the air in fragments of all sizes, 
large pieces of scoria, lapilli (fragments the size of a pea or 
walnut), volcanic " sand," and volcanic " ashes." The latter resem- 
ble in appearance the ashes of wood or coal, but they are not in 
any sense, like them, a residue after combustion. 

Volcanic ashes are produced in several ways : lava rising in 
the volcanic duct is exploded into fine dust by the steam which 
permeates it; glassy lava, hurled into the air and cooled sud- 
denly, is brought into a state of high strain and tension, and, 
like Prince Eupert's drops, flies to pieces at the least provocation. 
The clash of rising and falling projectiles also produces some 
dust, a fair sample of which may be made by grating together 
two pieces of pumice. 

Beds of volcanic ash occur widely among recent deposits in the. 
western United States. In Nebraska ash beds are found in twenty 
counties, and are often as white as powdered pumice. The beds grow 
thicker and coarser toward the southwestern part of the state, where 
their thickness sometimes reaches fifty feet. In what direction would 
you look for the now extinct volcano whose explosive eruptions are thus 
recorded ? 

Tuff. This is a convenient term designating any rock com- 
posed of volcanic fragments. Coarse tuffs of angular fragments 
are called volcanic breccia, and when the fragments have been 
rounded and sorted by water the rock is termed a volcanic con- 
glomerate. Even when deposited in the open air, as on the slopes 
of a volcano, tuffs may be rudely bedded and their fragments 
more or less rounded, and unless marine shells or the remains 
of land plants and animals are found as fossils in them, there is 
often considerable difficulty in telling whether they were laid 
in water or in air. In either case they soon become consolidated. 
Chemical deposits from percolating waters fill the interstices, 
and the bed of loose fragments is cemented to hard rock. 

The materials of which tuffs are composed are easily recog- 
nized as volcanic in their origin. The fragments are more or 



256 



THE ELEMENTS OF GEOLOGY 



less cellular, according to the degree to which they were dis- 
tended with steam when in a molten state, and even in the finest 
dust one may see the glass or the crystals of lava from which it 
was derived. Tuffs often contain volcanic bombs, balls of lava 
which took shape while whirling in the air, and solidified before 
falling to the ground. 

Ancient volcanic rocks. It is in these materials and struc- 
tures which we have described that volcanoes leave some of 

their most enduring 
records. Even the vol- 
canic rocks of the earli- 
est geological ages, up- 
lifted after long burial 
beneath the sea and ex- 
posed to view by deep 
erosion, are recognized 
and their history read 
despite the many changes 
which they may have 
undergone. A sheet of 
ancient lava may be distinguished by its composition from the 
sediments among which it is imbedded. The direction of its 
flow lines may be noted. The cellular and slaggy surface where 
the pasty lava was distended by escaping steam is recognized 
by the amygdules which now fill the ancient steam blebs. In 
a pile of successive sheets of lava each flow may be distinguished 
and its thickness measured ; for the surface of each sheet is 
glassy and scoriaceous, while beneath its upper portions the 
lava of each flow is more dense and stony. The length of time 
which elapsed before a sheet was buried beneath the materials 
of succeeding eruptions may be told by the amount of weather- 
ing which it had undergone, the depth of ancient soil now 
baked to solid rock upon it, and the erosion which it had 
suffered in the interval. 




FIG. 228. Volcanic Bombs, Cinder Cone, 
California 



VOLCANOES 



257 



If the flow occurred from some submarine volcano, we may 
recognize the fact by the sea-laid sediments which cover it, fill- 
ing the cracks and crevices of its upper surface and containing 
pieces of lava washed from it in their basal layers. 

Long-buried glassy lavas devitrify, or pass to a stony condi- 
tion, under the unceasing action of underground waters ; but 
their flow lines and perlitic and spherulitic structures remain 
to tell of their original 
state. 

Ancient tuffs are 
known by the frag- 
mental character of 
their volcanic material, 
even though they hare 
been altered to firm rock. 
Some remains of land 
animals and plants may 
be found imbedded to 
tell that the beds were laid in open air ; while the remains of 
marine organisms would prove as surely that the tuffs were 
deposited in the sea. 

In these ways ancient volcanoes have been recognized near 
Boston, in southeastern Pennsylvania, about Lake Superior, and 
in other regions of the United States. 




FIG. 229. A Volcanic Cone, Arizona 



THE LIFE HISTORY OF A VOLCANO 

The invasion of a region by volcanic forces is attended by 
movements of the crust heralded by earthquakes. A fissure or 
a pipe is opened and the building of the cone or the spreading 
of wide lava sheets is begun. 

Volcanic cones. The shape of a volcanic cone depends chiefly 
on the materials erupted. Cones made of fragments may have 
sides as steep as the angle of repose, which in the case of coarse 



258 



THE ELEMENTS OF GEOLOGY 



scoria is sometimes as high as thirty or forty degrees. About 
the base of the mountain the finer materials erupted are spread 
in more gentle slopes, and are also washed forward by rams and 
streams. The normal profile is thus a symmetric cone with a 
flaring base. 

Cones built of lava vary in form according to the liquidity 
of the lava. Domes of gentle slope, as those of Hawaii, for 




FIG. 230. Sarcoui, a Trachyte Dome, France 

example, are formed of basalt, which flows to long distances 
before it congeals. When superheated and emitted from many 
vents, this easily melted lava builds great plateaus, such as that 
of Iceland. On the other hand, lavas less fusible, or poured out 
at a lower temperature, stiffen when they have flowed but a 
short distance, and accumulate in a steep cone. Trachyte has 
been extruded in a state so viscid that it has formed steep- 
sided domes like that of Sarcoui (Fig. 230). 



VOLCANOES 



259 



Most volcanoes are built, like Vesuvius, both of lava flows and 
of tuffs, and sections show that the structure of the cone consists 
of outward-dipping, alternating layers of lava, scoria, and ashes. 




FIG. 231. Section of Vesuvius 

V, Vesuvius; S, Somma, a mountainous rampart half encircling Vesuvius, 
and like it built of outward-dipping sheets of tuff and lava ; a, crystalline 
rocks ; 6, marine strata; c, tuffs containing seashells. Which is the older 
mountain, Vesuvius or Somma ? Of what is Somma a remnant ? Draw 
a diagram showing its original outline. Suggest what processes may 
have brought" it to its present form. What record do you find of the 
earliest volcanic activity ? What do you infer as to the beginnings of 
the volcano ? 

From time to tune the cone is rent by the violence of explo- 
sions and by the weight of the column of lava in the pipe. 
The fissures are filled with lava and some discharge on the 
sides of the mountain, building parasitic cones, while all form 
dikes, which strengthen 
the pile with ribs of 
hard rock and make it 
more difficult to rend. 

Great catastrophes 




Scale of Miles. 
FIG. 232. Crater Lake, Oregon 



are recorded in the How wide and how deep is the basin which holds 

the lake ? The mountain walls which inclose 

Shape of some volcanoes it are made of outward-dipping sheets of lava. 

Draw a diagram restoring the volcano of which 
they are the remnant. No volcanic fragments 
of the same nature as the materials of which 
the volcano is built are found about the region. 
What theory of the destruction of the cone does 
this fact favor? W, Wizard Island, is a cinder 
cone. When was it built? 



which consist of a circu- 
lar run, perhaps miles 
in diameter, inclosing a 
vast crater or a caldera 



within which small 
cones may rise. We may infer that at some time the top of 
the mountain has been blown off, or has collapsed and been 
engulfed because some reservoir beneath had been emptied by 
long-continued eruptions (Fig. 232). 



260 



THE ELEMENTS OF GEOLOGY 



The cone-building stage may be said to continue until erup- 
tions of lava and fragmental materials cease altogether. Sooner 
or later the volcanic forces shift or die away, and no further 
eruptions add to the pile or replace its losses by erosion during 
periods of repose. .Gases however are still emitted, and, as sul- 
phur vapors are conspicuous among them, such vents are called 
solfataras. Mount Hood, in Oregon, is an example of a volcano 

sunk to this stage. From 
a steaming rift on its 
side there rise sulphur- 
'ous fumes which, half 
a mile down the wind, 
will tarnish a silver coin. 
Geysers and hot 
springs. The hot 
springs of volcanic re- 
gions are among the 
last vestiges of volcanic 
heat. Periodically erup- 
tive boiling springs are 
termed geysers. In each 
of the geyser regions of 
the earth the Yellow- 
stone National Park, 
Iceland, and New Zea- 
land the ground water 
of the locality is sup- 
posed to be heated by ancient lavas that, because of the poor 
conductivity of the rock, still remain hot beneath the surface. 

Old Faithful, one of the many geysers of the Yellowstone National 
Park, plays a fountain of boiling water a hundred feet in air; while 
clouds of vapor from the escaping steam ascend to several times that 
height. The eruptions take place at intervals of from seventy to ninety 
minutes. In repose the geyser is a quiet pool, occupying a craterlike 




FIG. 233. Old Faithful Geyser in Eruption, 
Yellowstone National Park 



VOLCANOES 



261 



depression in a conical mound some twelve feet high. The conduit of 
the spring is too irregular to be sounded. The mound is composed of 
porous silica deposited by the waters of the geyser. 

Geysers erupt at intervals instead of continuously boiling, 
because their long, narrow, and often tortuous conduits do not 
permit a free circulation of the water. After an eruption the 
tube is refilled and the water again gradually becomes heated 




FIG. 234. Terrace and Cones of Siliceous Sinter deposited by Geysers, 
Yellowstone National Park 

Deep in the tube where it is in contact with hot lavas the 
water sooner or later reaches the boiling point, and bursting 
into steam shoots the water above it high in air. 

Carbonated springs. After all the other signs of life have 
gone, the ancient volcano may emit carbon dioxide as its dying 
breath. The springs of the region may long be charged with 
carbon dioxide, or carbonated, and where they rise through 
limestone may be expected to deposit large quantities of traver- 
tine. We should remember, however, that many carbonated 
springs, and many hot springs, are wholly independent of 
volcanoes. 



262 



THE ELEMENTS OF GEOLOGY 



The destruction of the cone. As soon as the volcanic cone 
ceases to grow by eruptions the agents of erosion begin to wear 




FIG. 235. Mount Shasta, California 

it down, and the length of time that has elapsed since the period 
of active growth may be roughly measured by the degree to 
which the cone has been dissected. We infer that Mount Shasta, 




FIG. 236. Mount Hood, Oregon 

whose conical shape is still preserved despite the gullies one 
thousand feet deep which trench its sides (Fig. 235), is younger 
than Mount Hood, which erosive agencies have carved to a 



VOLCANOES 



263 



pyramidal form (Fig. 236). The pile of materials accumulated 
about a volcanic vent, no matter how vast in bulk, is at last 




Scale of Miles 
FIG. 237. Crandall Volcano 

swept entirely away. The cone of a volcano, active or extinct, 
is not old as the earth counts tune ; volcanoes are short-lived 
geological phenomena. 

Crandall Volcano. This 
name is given to a dis- 
sected ancient volcano in 
the Yellowstone National 
Park, which once, it is 
estimated, reared its head 
thousands of feet above the 
surrounding country and 
greatly exceeded in bulk 
either Mount Shasta or 
Mount Etna. Not a line 
of the original mountain 
remains; all has been swept 
away by erosion except 
some four thousand feet of 
the base of the pile. This 
basal wreck now appears 
as a rugged region about 
thirty miles in diameter, 
trenched by deep valleys 
and cut into sharp peaks 
and precipitous ridges. In 
the center of the area is found the nucleus (A 7 , Fig. 237), a mass of 
coarsely crystalline rock that congealed deep in the old volcanic pipe. 
From it there radiate in all directions, like the spokes of a wheel, long 
dikes whose rock grows rapidly finer of grain as it leaves the vicinity of 




FIG. 238. Fossil Tree Trunks, Yellowstone 

National Park 
To the left is seen a mass of volcanic breccia 



264 THE ELEMENTS OF GEOLOGY 

the once heated core. The remainder of the base of the ancient moun- 
tain is made of rudely bedded tuffs and volcanic breccia, with occasional 
flows of lava, some of the fragments of the breccia measuring as much 
as twenty feet in diameter. On the sides of canyons the breccia is 
carved by rain erosion to fantastic pinnacles. At different levels in the 
midst of these beds of tuff and lava are many old forest grounds. The 
stumps and trunks of the trees, now turned to stone, still in many cases 
stand upright where once they grew on the slopes of the mountain as it 
was building (Fig. 238). The great size and age of some of these trees 
indicate the lapse of time between the eruption whose lavas or tuffs 
weathered to the soil on which they grew and the subsequent eruption 
which buried them beneath showers of stones and ashes. 

Near the edge of the area lies Death Gulch, in which carbon dioxide 
is given off in such quantities that in quiet weather it accumulates in a 
heavy layer along the ground and suffocates the animals which may 
enter it. 



CHAPTEE XII 
UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 

It is because long-continued erosion lays bare the innermost 
anatomy of an extinct volcano, and even sweeps away the 
entire pile with much of the underlying strata, thus leaving 
the very roots of the volcano open to view, that we are able to 
study underground volcanic structures. With these we include, 
for convenience, intrusions of molten rock which have been 
driven upward into the crust, but which may not have suc- 
ceeded in breaking way to the surface and establishing a vol- 
cano. All these structures are built of rock forced when in a 
fluid or pasty state into some cavity which it has found or made, 
and we may classify them therefore, according to the shape of 
the molds in which the molten rock has congealed, as (1) dikes, 
(2) volcanic necks, (3) intrusive sheets, and (4) intrusive masses. 

Dikes. The sheet of once molten rock with which a fissure 
has been filled is known as a dike. Dikes are formed when 
volcanic cones are rent by explosions or by the weight of the 
lava column hi the duct, and on the dissection of the pile they 
appear as radiating vertical ribs cutting across the layers of 
lava and tuff of which the cone is built. In regions under- 
going deformation rocks lying deep below the ground are often 
broken and the fissures are filled with molten rock from beneath, 
which finds no outlet to the surface. Such dikes are common 
in areas of the most ancient rocks, which have been brought to 
light by long erosion. 

In exceptional cases dikes may reach the length of fifty or 
one hundred miles. They vary in width from a fraction of a 
foot to even as much as three hundred feet. 

265 





r 



206 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 267 

Dikes are commonly more fine of grain on the sides than in the 
center, and may have a glassy and crackled surface where they meet 
the inclosing rock. Can you account for this on any principle which 
you have learned ? 

Volcanic necks. The pipe of a volcano rises from far below 
the base of the cone, from the deep reservoir from which its 



N\ 



Fi<;. 240. A Dissected Volcanic Cone 

N, volcanic neck ; I, I, lava-topped table mountains ; t, t, beds of tuff ; d, d, 
dikes ; dotted lines indicate the initial profile 

eruptions are supplied. When the volcano has become extinct 
this great tube remains filled with hardened lava. It forms a 
cylindrical core of solid rock, except for some distance below 
the ancient crater, where it may contain a mass of fragments 
which had fallen back into the chimney after being hurled into 
the air. 

As the mountain is worn down, this central column known 
as the volcanic neck is left standing as a conical hill (Fig. 240). 
Even when every other 
trace of the volcano has 
been swept away, ero- 
sion will not have passed 
below this great stalk on 
which the volcano was 
borne as a fiery flower 
whose site it remains 

to mark. In volcanic FlG ' 241 ' Mount Johnson > a Volcanic Neck 

near Montreal 
regions of deep denuda- 
tion volcanic necks rise solitary and abrupt from the surround- 
ing country as dome-shaped hills. They are marked features in 




268 



THE ELEMENTS OF GEOLOGY 



the landscape in parts of Scotland and in the St. Lawrence val- 
ley about Montreal (Fig. 241). 

Intrusive sheets. Sheets of igneous rocks are sometimes 
found interleaved with sedimentary strata, especially in regions 
where the rocks have been deformed and have suffered from 
volcanic action. In some instances such a sheet is seen to be 
contemporaneous (p. 248). In other instances the sheet must 




FIG. 242. The Palisades of the Hudson, New Jersey 

be intrusive. The overlying stratum, as well as that beneath, 
has been affected by the heat of the once molten rock. We 
infer that the igneous rock when in a molten state was forced 
between the strata, much as a card may be pushed between the 
leaves of a closed book. The liquid wedged its way between 
the layers, lifting those above to make room for itself. The 
source of the intrusive sheet may often be traced to some 
dike (known therefore as the feeding dike), or to some mass of 
igneous rock. 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 269 



Intrusive sheets may extend a score and more of miles, and, 
like the longest surface flows, the most extensive sheets consist 
of the more fusible and fluid lavas, those of the basic class of 
which basalt is an example. Intrusive sheets are usually harder 
than the strata in which they lie and are therefore often left in 
relief after long denudation of the region (Fig. 315). 

On the west bank of the Hudson there extends from New York Bay 
north for thirty miles a bold cliff several hundred feet high, the 
Palisades of the Hudson. It is 
the outcropping edge of a sheet 
of ancient igneous rock, which 




FIG. 243. Diagram of the Palisades of 
the Hudson 

i, intrusive sheet; s, sandstone; d, feeding 
dike; HR, Hudson River 



rests on stratified sandstones 

and is overlain by strata of the 

same series. Sandstones and 

lava sheet together dip gently 

to the west and the latter disappears from view two miles back from 

the river. 

It is an interesting question whether the Palisades sheet is contem- 
poraneous or intrusive. Was it outpoured on the sandstones beneath it 
when they formed the floor of the sea, and covered forthwith by the 
sediments of the strata above, or was it intruded among these beds at a 

later date ? 

The latter is the 
case ; for the overly- 
ing stratum is in- 

Scale of Miles tensely baked along 

FIG. 244. Section of Electric Peak, E, and Gray the zone of contact. 
Peak, G, Yellowstone National Park ^ the west edge of 

Intrusive sheets and masses of igneous rock are drawn the sheet is found the 
in black dike in which the lava 

rose to force its way far and wide between the strata. 

Electric Peak, one of the prominent mountains of the Yellowstone 
National Park, is carved out of a mass of strata into which many 
sheets of molten rock have been intruded. The western summit con- 
sists of such a sheet several hundred feet thick. Studying the section 
of Figure 244, what inference do you draw as to the source of these 
intrusive sheets? 




270 



THE ELEMENTS OF GEOLOGY 




INTRUSIVE MASSES 

Bosses. This iiame is generally applied to huge irregular 
masses of coarsely crystalline igneous rock lying in the midst 

of other formations. Bosses 
vary greatly in size and may 
reach scores of miles in ex- 
tent. Seldom are there any 
evidences found that bosses 
ever had connection with 
the surface. On the other 
hand, it is often proved that 
they have been driven, or 
FIG. 245. Stone Mountain, Georgia, a have melted their way, up- 
ward into the formations in 

which they lie ; for they give off dikes and intrusive sheets, and 
have profoundly altered the rocks about them by their heat. 

The texture of the rock 
of bosses proves that con- 
solidation proceeded slowly 
and at great depths, and it 
is only because of vast de- 
nudation that they are now 
exposed to view. Bosses are 
commonly harder than the 
rocks about them, and stand 
up, therefore, as rounded 
hills and mountainous 
ridges long after the sur- 
rounding country has worn 
to a low plain (Fig. 245). 
The base of bosses is in- 




definite or undetermined, v 

FIG. 246. Map of Granite Bosses near 

and in this respect they Baltimore (areas horizontally lined) 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 271 



differ from laccoliths. Some bosses have broken and faulted 
the overlying beds; some have forced the rocks aside and 
melted them away. 

The Spanish Peaks of southeastern Colorado were formed by the 
upthrust of immense masses of igneous rock, bulging and breaking the 
overlying strata. On one side of the mountains the throw of the fault 
is nearly a mile, and fragments of deep-lying beds were dragged upward 
by the rising masses. The adjacent rocks were altered by heat to a 
distance of several thousand feet. No evidence appears that the molten 
rock ever reached the surface, and if volcanic eruptions ever took place 
either in lava flows or fragmental materials, all traces of them have 
been effaced. The rock of the intrusive masses is coarsely crystalline, 
and no doubt solidified slowly under the pressure of vast thicknesses of 
overlying rock, now mostly removed by erosion. 

A magnificent system of dikes radiates from the Peaks to a distance 
of fifteen miles, some now being left by long erosion as walls a hundred 
feet in height (Fig. 239). Intrusive sheets fed by the dikes penetrate 
the surrounding strata, and their edges are cut by canyons as much as 
twenty-five miles from the mountain. In these strata are valuable beds 
of lignite, an imperfect coal, which the heat of dikes and sheets has 
changed to coke. 

Laccoliths. The laccolith (Greek laccos, cistern; lithos, stone) 
is a variety of intrusive masses in which molten rock has 
spread between the strata, 
and, lifting the strata above 
it to a dome-shaped form, 
has collected beneath them 
in a lens-shaped body with 
a flat base. 

The Henry Mountains, a 

, , , , , , FIG. 247. Section of a Laccolith 

small group of detached peaks 

in southern Utah, rise from a plateau of horizontal rocks. Some of the 
peaks are carved wholly in separate domelike uplifts of the strata of 
the plateau. In others, as Mount Killers, the largest of the group, there 
is exposed on the summit a core of igneous rock from which the sedi- 
mentary rocks of the flanks dip steeply outward in all directions. In 





272 THE ELEMENTS OF GEOLOGY 

still others erosion has stripped oft' the covering strata and has laid bare 
the core to its base ; and its shape is here seen to be that of a plano- 
convex lens or a baker's bun, its flat base resting on the undisturbed 
bedded rocks beneath. The structure of Mount Killers is shown in 
Figure 248. The nucleus of igneous rock is four miles in diameter 
and more than a mile in depth. 

Regional intrusions. These vast bodies of igneous rock, which 
may reach hundreds of miles in diameter, differ little from bosses 
except in their immense bulk. Like bosses, regional intrusions 

give off dikes and sheets 
and greatly change the 
rocks about them by 
their heat. They are 
now exposed to view 
only because of the pro- 
FIG. 248. Section of Mount Killers found denudation which 

has removed the upheaved dome of rocks beneath which they 
slowly cooled. Such intrusions are accompanied whether as 
cause or as effect is still hardly known by deformations, and 
their masses of igneous rock are thus found as the core of many 
great mountain ranges. The granitic masses of which the Bitter 
Root Mountains and the Sierra Nevadas have been largely carved 
are each more than three hundred miles in length. Immense 
regional intrusions, the cores of once lofty mountain ranges, are 
found upon the Laurentian peneplain. 

Physiographic effects of intrusive masses. We have already 
seen examples of the topographic effects of intrusive masses in 
Mount Killers, the Spanish Peaks, and in the great mountain 
ranges mentioned in the paragraph on regional intrusions, 
although in the latter instances these effects are entangled 
with the effects of other processes. Masses of igneous rock 
cannot be intruded within the crust without an accompanying 
deformation on a scale corresponding to the bulk of the in- 
truded mass. The overlying strata are arched into hills or 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 273 

mountains, or, if the molten material is of great extent, the strata 
may Conceivably be floated upward to the height of a plateau. 
We may suppose that the transference of molten matter from one 
region to another may be among the causes of slow subsidences 
and elevations. Intrusions give rise to fissures, dikes, and in- 
trusive sheets, and these dislocations cannot fail to produce earth- 
quakes. Where intrusive masses open communication with the 
surface, volcanoes are established or fissure eruptions occur such 
as those of Iceland. 

THE INTRUSIVE EOCKS 

The igneous rocks are divided into two general classes, the 
volcanic or eruptive rocks, which have been outpoured in open 
air or on the floor of the sea, and the intrusive rocks, which 
have been intruded within the rocks of the crust and have solid- 
ified below the surface. The two classes are alike in chemical 
composition and may be divided into acidic and basic groups. 
In texture the intrusive rocks differ from the volcanic rocks 
because of the different conditions under which they have 
solidified. They cooled far more slowly beneath the cover of 
the rocks into which they were pressed than is permitted to lava 
flows in open air. Their constituent minerals had ample oppor- 
tunity to sort themselves and crystallize from the fluid mixture, 
and none of that mixture was left to congeal as a glassy paste. 

They consolidated also under pressure. They are never sco- 
riaceous, for the steam with which they were charged was not 
allowed to expand and distend them with steam blebs. In the 
rocks of the larger intrusive masses one may see with a power- 
ful microscope exceedingly minute cavities, to be counted by 
many millions to the cubic inch, in which the gaseous water 
which the mass contained was held imprisoned under the im- 
mense pressure of the overlying rocks. 

Naturally these characteristics are best developed in the 
intrusives which cooled most slowly, i.e. in the deepest-seated 



274 THE ELEMENTS OF GEOLOGY 

and largest masses ; while in those which cooled more rapidly, 
as in dikes and sheets, we find gradations approaching the 
texture of surface flows. 

Varieties of the intrusive rocks. We will now describe a 
few of the varieties of rocks of deep-seated intrusions. All are 
even grained, consisting of a mass of crystalline grains formed 
during one continuous stage of solidification, and no porphyritic 
crystals appear as in lavas. 

Granite, as we have learned already, is composed of three 
minerals, quartz, feldspar, and mica. According to the color of 
the feldspar the rock may be red, or pink, or gray. Hornblende 
a black or dark green mineral, an iron-magnesian silicate, 
about as hard as feldspar is sometimes found as a fourth 
constituent, and the rock is then known as hornblendic granite, 
Granite is an acidic rock corresponding to rhyolite in chemical 
composition. We may believe that the same molten mass which 
supplies this acidic lava in surface flows solidifies as granite 
deep below ground in the volcanic reservoir. 

Syenite, composed of feldspar and mica, has consolidated 
from a less siliceous mixture than has granite. 

Diorite, still less siliceous, is composed of hornblende and 
feldspar, 1 - the latter mineral being of different variety from the 
feldspar of granite and syenite. 

Galibro, a typical basic rock, corresponds to basalt in chemical 
composition. It is a dark, heavy, coarsely crystalline aggregate 
of feldspar and auyite (a dark mineral allied to hornblende). It 
often contains magnetite (the magnetic black oxide of iron) and 
olivine (a greenish magnesian silicate). 

In the northern states all these types, and many others also 
of the vast number of varieties of intrusive rocks, can be found 
among the rocks of the drift brought from the areas of igneous 
rock in Canada and the states of our northern border. 

Summary. The records of geology prove that since the earli- 
est of their annals tremendous forces have been active iii the 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 275 

earth. In all the past, under pressures inconceivably great, 
molten rock has been driven upward into the rocks of the crust. 






FIG. 249. Ground Plan of Dikes 
in Granite. (Scale 80 feet to 
the inch) 

What is the relative age of the dikes 
aa, bb, and cc ? 



FIG. 250, A and B. Mountains 
of coarsely Crystalline Ig- 
neous Rock i, surrounded 
by Sedimentary Strata s 
ands' 

Copy each diagram and complete 
it, so as to show whether the 
mass of igneous rock is a 
volcanic neck, a boss, or a 
laccolith 



It has squeezed into fissures forming dikes; it has burrowed 
among the strata as intrusive sheets ; it has melted the rocks 
away or lifted the overlying strata, filling the chambers which 
it has made with intrusive masses. During 
all geological ages molten rock has found 
way to the surface, and volcanoes have 
darkened the sky with clouds of ashes and 
poured streams of glowing lava down their 



FIG. 251. 

1, limestone; 2, tuff; 
3, 5, 7, shale with 
marine shells; 4, 6, 
lava, dotted portions 
scoriaceous. Give the 
history recorded in 
this section 





FIG. 252. 

a, sedimentary strata with intrusive sheets; b, sedi- 
mentary strata; c, lava flow; d, dike. Give the 
succession of events recorded in this section 



276 



THE ELEMENTS OF GEOLOGY 



sides. The older strata, the strata which have been most 
deeply buried, and especially those which have suffered most 
from folding and from fracture, show the largest amount of igne- 
ous intrusions. The molten rock which has been 
driven from the earth's interior to within the 
crust or to the surface during geologic time must 
be reckoned in millions of cubic miles. 

IB 




FIG. 253 

Which of the lava sheets of 
this section are contem- 
poraneous and which in- 
trusive, A, whose upper 
surface is overlain with 
a conglomerate of rolled 
lava pebbles ; B, the cracks 
and seams of whose upper 
surface are filled with the 
material of the overly- 
ing sandstone ; C, which 
breaks across the strata in 
which it is imbedded ; D, 
which includes fragments 
of both the underlying 
and overlying strata and 
penetrates their crevices 
and seams? 




FIG. 2;">4. Mato Tepee, Wyoming 
This magnificent tower of igneous rock three 
hundred feet in height has been called by 
some a volcanic neck. Is the direction of the 
columns that which would obtain in the 
cylindrical pipe of a volcano? The tower is 
probably the remnant of a small laccolith, an 
outlying member of a group of laccoliths 
situated not far distant 



THE INTERIOR CONDITION OF THE EARTH AND CAUSES 
OF VULCANISM AND DEFORMATION 

The problems of volcanoes and of deformation are so closely 
connected witli that of the earth's interior that we may consider 
them together. Few of these problems are solved, and we may 
only state some known facts and the probable conclusions 
which may be drawn as inferences from them. 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 277 

The interior of the earth is hot. Volcanoes prove that in 
many parts of the earth there exist within reach of the sur- 
face regions of such intense heat that the rock is in a molten 
condition. Deep wells and mines show everywhere an increase 
in temperature below the surface shell affected by the heat of 
summer and the cold of winter, a shell in temperate latitudes 
sixty or seventy feet thick. Thus in a boring more than a mile 
deep at Schladebach, Germany, the earth grows warmer at the 
rate of 1 F. for every sixty-seven feet as we descend. Taking 
the average rate of increase at one degree for every sixty feet 
of descent, and assuming that this rate, observed at the moderate 
distances open to observation, continues to at least thirty-five 
miles, the temperature at that depth must be more than three 
thousand degrees, a temperature at which all ordinary rocks 
would melt at the earth's surface. The rate of increase in tem- 
perature probably lessens as we go downward, and it may not be 
appreciable below a few hundred miles. But there is no reason 
to doubt that the interior of the earth is intensely hot. Below 
a depth of one or two score miles we may imagine the rocks 
everywhere glowing with heat. 

Although the heat of the ulterior is great enough to melt all 
rocks at atmospheric pressure, it does not follow that the interior 
is fluid. Pressure raises the fusing point of rocks, and the 
weight of the crust may keep the interior in what may be 
called a solid state, although so hot as to be a liquid or a gas 
were the pressure to be removed. 

The interior of the earth is rigid and heavy. The earth 
behaves as a globe more rigid than glass under the attractions 
of the sun and moon. It is not deformed by these stresses as 
is the ocean hi the tides, proving that it is not a fluid ball cov- 
ered with a yielding crust a few miles thick. Earthquakes pass 
through the earth faster than they would were it of solid steel. 
Hence the rocks of the interior are highly elastic, being brought 
by pressure to a compact, continuous condition unbroken by 



278 THE ELEMENTS OF GEOLOGY 

the cracks and vesicles of surface rocks. The interior of the 
earth is rigid. 

The common rocks of the crust are about two and a half 
times heavier than water, while the earth as a whole weighs 
five and six-tenths times as much as a globe of water of the same 
size. The interior is therefore much more heavy than the crust. 
This may be caused in part by compression of the interior 
under the enormous weight of the crust, and in part also by 
an assortment of material, the heavier substances, such as the 
heavy metals, having gravitated towards the center. 

Between the crust, which is solid because it is cool, and the 
interior, which is hot enough to melt were it not for the pressure 
which keeps it dense and rigid, there may be an intermediate 
zone in which heat and pressure are so evenly balanced that 
here rock liquefies whenever and wherever the pressure upon 
it may be relieved by movements of the crust. It is perhaps 
from such a subcrustal layer that the lava of volcanoes is 
supplied. 

The causes of volcanic action. It is now generally believed 
that the heat of volcanoes is that of the earth's interior. Other 
causes, such as friction and crushing in the making of moun- 
tains and the chemical reactions between oxidizing agents of 
the crust and the unoxidized interior, have been suggested, but 
to most geologists they seem inadequate. 

There is much difference of opinion as to the force which 
causes molten rock to rise to the surface in the ducts of vol- 
canoes. Steam is so evidently concerned in explosive eruptions 
that many believe that lava is driven upward by the expansive 
force of the steam with which it is charged, much as a viscid 
liquid rises and boils over in a test tube or kettle. 

But in quiet eruptions, and still more in the irruption of intru- 
sive sheets and masses, there is little if any evidence that steam 
is the driving force. It is therefore believed by many geologists 
that it is pressure due to crustal movements and internal stresses 



UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN 279 

which squeezes molten rock from below into fissures and ducts 
in the crust. It is held by some that where considerable water 
is supplied to the rising column of lava, as from the ground 
water of the surrounding region, and where the lava is viscid 
so that steam does not readily escape, the eruption is of the 
explosive type ; when these conditions do not obtain, the lava 
outwells quietly, as in the Hawaiian volcanoes. It is held by 
others not only that volcanoes are due to the outflow of the 
earth's deep-seated heat, but also that the steam and other 
emitted gases are for the most part native to the earth's in- 
terior and never have had place in the circulation of atmos- 
pheric and ground waters. 

Volcanic action and deformation. Volcanoes do not occur on 
wide plains or among ancient mountains. On the other hand, 
where movements of the earth's crust are in progress in the 
uplift of high plateaus, and still more in mountain making, 
molten rock may reach the surface, or may be driven upward 
toward it forming great intrusive masses. Thus extensive lava 
flows accompanied the upheaval of the block mountains of west- 
ern North America and the uplift of the Colorado plateau. A 
line of recent volcanoes may be traced along the system of rift 
valleys which extends from the Jordan and Dead Sea througli 
eastern Africa to Lake Nyassa. The volcanoes of the Andes 
show how conspicuous volcanic action may be in young rising 
ranges. Folded mountains often show a core of igneous rock, 
which by long erosion has come to form the axis and the highest 
peaks of the range, as if the molten rock had been squeezed up 
under the rising upfolds. As we decipher the records of the 
rocks in historical geology we shall see more fully how, in all 
the past, volcanic action has characterized the periods of great 
crustal movements, and how it has been absent when and where 
the earth's crust has remained comparatively at rest. 

The causes of deformation. As the earth's interior, or nucleus, 
is highly heated it must be constantly though slowly losing its 



280 THE ELEMENTS OF GEOLOGY 

heat by conduction through the crust and into space ; and since 
the nucleus is cooling it must also be contracting. The nucleus 
has contracted also because of the extrusion of molten matter, 
the loss of constituent gases given off in volcanic eruptions, and 
(still more important) the compression and consolidation of its 
material under gravity. As the nucleus contracts, it tends to 
draw away from the cooled and solid crust, and the latter set- 
tles, adapting itself to the shrinking nucleus much as the skin 
of a withering apple wrinkles down upon the shrunken fruit. 
The unsupported weight of the spherical crust develops enor- 
mous tangential pressures, similar to the stresses of an arch 
or dome, and when these lateral thrusts accumulate beyond 
the power of resistance the solid rock is warped and folded and 
broken. 

Since the planet attained its present mass it has thus been 
lessening in volume. Notwithstanding local and relative up- 
heavals the earth's surface on the whole has drawn nearer and 
nearer to the center. The portions of the lithosphere which 
have been carried down the farthest have received the waters 
of the oceans, while those portions which have been carried 
down the least have emerged as continents. 

Although it serves our convenience to refer the movements 
of the crust to the sea level as datum plane, it is understood 
that this level is by no means fixed. Changes in the ocean 
basins increase or reduce their capacity and thus lower or raise 
the level of the sea. But since these basins are connected, the 
effect of any change upon the water level is so distributed that 
it is far less noticeable than a corresponding change would be 
upon the land. 



CHAPTER XIII 
METAMORPHISM AND MINERAL VEINS 

Under the action of internal agencies rocks of all kinds may 
be rendered harder, more firmly cemented, and more crystalline. 
These processes are known as metamorphism , and the rocks 
affected, whether originally sedimentary or igneous, are called 
metamorphic rocks. We may contrast with metamorphism the 
action of external agencies in weathering, which render rocks 
less coherent by dissolving their soluble parts and breaking 
down their crystalline grams. 

Contact metamorphism. Rocks beneath a lava flow or in 
contact with igneous intrusions are found to be metamorphosed 
to various degrees by the heat of the cooling mass. The adja- 
cent strata may be changed only in color, hardness, and texture. 
Thus, next to a dike, bituminous coal may be baked to coke or 
anthracite, and chalk and limestone to crystalline marble. Sand- 
stone may be converted into quartzite, and shale into argillite, 
a compact, massive clay rock. New minerals may also be de- 
veloped. In sedimentary rocks there may be produced crystals 
of mica and of garnet (a mineral as hard as quartz, commonly 
occurring in red, twelve-sided crystals). Where the changes are 
most profound, rocks may be wholly made over in structure and 
mineral composition. 

In contact metamorphism thin sheets of molten rock pro- 
duce less effect than thicker ones. The strongest heat effects 
are naturally caused by bosses and regional intrusions, and the 
zone of change about them may be several miles in width. In 
these changes heated waters and vapors from the masses of 
igneous rocks undoubtedly play a very important part. 

281 



282 THE ELEMENTS OF GEOLOGY 

Which will be more strongly altered, the rocks about a closed dike 
in which lava began to cool as soon as it filled the fissure, or the rocks 
about a dike which opened on the surface and through which the 
molten rock flowed for some time? 

Taking into consideration the part played by heated waters, which will 
produce the most far-reaching metamorphism, dikes which cut across the 
bedding planes or intrusive sheets which are thrust between the strata? 

Regional metamorphism. Metamorphic rocks occur wide- 
spread in many regions, often hundreds of square miles in area, 
where such extensive changes cannot be accounted for by 
igneous intrusions. Such are the dissected cores of lofty moun- 
tains, as the Alps, and the worn-down bases of ancient ranges, 
as in New England, large areas in the Piedmont Belt, and the 
Laurentian peneplain. 

. In these regions the rocks have yielded to immense pressure. 
They have been folded, crumpled, and mashed, and even their 
minute grains, as one may see with a microscope, have often 
been puckered, broken, and crushed to powder. It is to these 
mechanical movements and strains which the rocks have suf- 
fered in every part that we may attribute their metamorphism, 
and the degree to which they have been changed is in direct pro- 
portion to the degree to which they have been deformed and 
mashed. 

Other factors, however, have played important parts. Rock 
crushing develops heat, and allows a freer circulation of heated 
waters and vapors. Thus chemical reactions are greatly quick- 
ened ; minerals are dissolved and redeposited in new positions, 
or their chemical constituents may recombine in new minerals, 
entirely changing the nature of the rock, as when, for example, 
feldspar recrystallizes as quartz and mica. 

Early stages of metamorphism are seen in slate. Pressure has 
hardened the marine muds, the arkose (p. 186), or the volcanic ash 
from which slates are derived, and has caused them to cleave by the 
rearrangement of their particles. 



METAMORPHISM AND MINERAL VEINS 283 

Under somewhat greater pressure, slate becomes phyllite, a clay slate 
whose cleavage surfaces are lustrous with flat-lying mica flakes. The 
same pressure which has caused the rock to cleave has set free some of 
its mineral constituents along the cleavage planes to crystallize there 
as mica. 

Foliation. Under still stronger pressure the whole structure 
of the rock is altered. The minerals of which it is composed, 
and the new miner- 
als which develop 
by heat and pres- 
sure, arrange them- 
selves along planes 
of cleavage or of 
shear in rudely par- 
allel leaves, or folia. 
Of this structure, 
called foliation, we 
may distinguish two 
types, a coarser 
f eld spathic type, 
and a fine type in 
which other miner- 
als than feldspar 
predominate. 

Gneiss is the 

FIG. 255. A Foliated Rock 
general name under 

which are comprised coarsely foliated rocks banded with irregu- 
lar layers of feldspar and other minerals. The gneisses appear 
to be due in many cases to the crushing and shearing of deep- 
seated igneous rocks, such as granite and gabbro. 

The crystalline schists, representing the finer types of folia- 
tion, consist of thin, parallel, crystalline leaves, which are often 
remarkably crumpled. These folia can be distinguished from 
the laminae of sedimentary rocks by their lenticular form and 




284 THE ELEMENTS OF GEOLOGY 

lack of continuity, and especially by the fact that they consist 
of platy, crystalline grains, and not of particles rounded by 
wear. 

Mica schist, the most common of schists, and in fact of all metamor- 
phic rocks, is composed of mica and quartz in alternating wavy folia. 
All gradations between it and phyllite may be traced, and in many 
cases we may prove it due to the metarnorphism of slates and shales. 
It is widespread in New England and along the eastern side of the 
Appalachians. Talc schist consists of quartz and talc, a light-colored 
magnesian mineral of greasy feel, and so soft that it can be scratched 
with the thumb nail. 

Hornblende schist, resulting in many cases from the foliation of basic 
igneous rocks, is made of folia of hornblende alternating with bands 
of quartz and feldspar. Hornblende schist is common over large areas 
in the Lake Superior region. 

Quartz schist is produced from quartzite by the development of fine 
folia of mica along planes of shear. All gradations may be found 
between it and unfoliated quartzite on the one hand and mica schist on 
the other. 

Under the resistless pressure of crustal movements almost any rocks, 
sandstones, shales, lavas of all kinds, granites, diorites, and gabbros 
may be metamorphosed into schists by crushing and shearing. Lime- 
stones, however, are metamorphosed by pressure into marble, the grains 
of carbonate of lime recrystallizing freely to interlocking crystals of 
calcite. 

These few examples must suffice of the great class of meta- 
morphic rocks. As we have seen, they owe their origin to the 
alteration of both of the other classes of rocks the sedimentary 
and the igneous by. heat and pressure, assisted usually by 
the presence of water. The fact of change is seen in their hard- 
ness and cementation, their more or less complete recrystalli- 
zation, and their foliation ; but the change is often so complete 
that no trace of their original structure and mineral composi- 
tion remains to tell whether the rocks from which they were 
derived were sedimentary or igneous, or to what variety of either 
of these classes they belonged. 




285 



286 



THE ELEMENTS OF GEOLOGY 



In many cases, however, the early history of a metamorpliic 
rock can be deciphered. Fossils not wholly obliterated may 
prove it originally water-laid. Schists may contain rolled-out 
pebbles, showing their derivation from a conglomerate. Dikes 
of igneous rocks may be followed into a region where they have 
been foliated by pressure. The most thoroughly metamorphosed 
rocks may sometimes be traced out into unaltered sedimentary 
or igneous rocks, or among them may be found patches of little 

change where their 
history maybe read. 
Metamorphism 
is most common 
among rocks of the 
earlier geological 
ages, and most rare 
among rocks of 
recent formation. 
No doubt it is now 
in progress where 

deep-buried sedi- 
FIG. 257. Quartz Veins in Slate 

ments are invaded 

by heat either from intrusive igneous masses or from the earth's 
interior, or are suffering slow deformation under the thrust of 
mountain-making forces. 

Suggest how rocks now in process of metamorphism may sometimes 
be exposed to view. Why do metamorphic rocks appear on the surface 
to-day? 

MINERAL VEINS 

In regions of folded and broken rocks fissures are frequently 
found to be filled with sheets of crystalline minerals deposited 
from solution by underground water, and fissures thus filled are 
known as mineral veins. Much of the importance of mineral 
veins is due to the fact that they are often metalliferous, 





METAMORPHISM AND MINERAL VEINS 287 

carrying valuable native metals and metallic ores disseminated 
in fine particles, in strings, and sometimes in large masses in the 
midst of the valueless nonmetallic minerals which make up 
what is known as the vein stone. 

The most common vein stones are quartz and calcite. Fluorite (cal- 
cium fluoride), a mineral harder than calcite and crystallizing in cubes 
of various colors, and barite (barium sulphate), a heavy white mineral, 
are abundant in many veins. 

The gold-bearing quartz veins of California traverse the metamor- 
phic slates of the Sierra Nevada Mountains. Below the zone of solution 
(p. 45) these veins consist of a vein stone of quartz mingled with 
pyrite (p. 13), the latter containing threads and grains of native gold. 
But to the depth of about 
fifty feet from the surface 
the pyrite of the vein has 
been dissolved, leaving a 

rusty, cellular quartz with 

FIG. 258. Placer Deposits in California 
grains of the insoluble gold 

scattered through it. 9, gold-bearing gravels in present river beds ; g>, 

. ancient gold-bearing river gravels ; a, a, lava 

The placer deposits of fl ows capping table mountains ; s, slate. Draw 

California and other a diagram showing by dotted lines conditions 

regions are gold-bearing before the lava flows occurred. What changes 

have since taken place ? 
deposits of gravel and sand 

in river beds. The heavy gold is apt to be found mostly near or upon the 
solid rock, and its grains, like those of the sand, are always rounded. 
How the gold came in the placers we may leave the pupil to suggest. 

Copper is found in a number of ores, and also in the native 
metal. Below the zone of surface changes the ore of a cop- 
per vein is often a double sulphide of iron and copper called 
clialcopyrite, a mineral softer than pyrite it can easily be 
scratched with a knife arid deeper yellow in color. For sev- 
eral score of feet below the ground the vein may consist of 
rusty quartz from which the metallic ores have been dissolved ; 
but at the base of the zone of solution we may find exceedingly 
rich deposits of copper ores, copper sulphides, red and black 
copper oxides, and green and blue copper carbonates, which 



288 THE ELEMENTS OF GEOLOGY 

have clearly been brought down in solution from the leached 
upper portion of the vein. 

Origin of mineral veins. Both vein stones and ores have been 
deposited slowly from solution in water, much as crystals of salt 
are deposited on the sides of a jar of saturated brine. In our 
study of underground water we learned that it is everywhere 
circulating through the permeable rocks of the crust, descend- 
ing to profound depths under the action of gravity and again 
driven to the surface by hydrostatic pressure. Now fissures, 
wherever they occur, form the trunk channels of the under- 
ground circulation. Water descends from the surface along 
these rifts ; it moves laterally from either side to the fissure 
plane, just as ground water seeps through the surrounding rocks 
from every direction to a well ; and it ascends through these 
natural water ways as in an artesian well, whenever they inter- 
sect an aquifer in which water is under hydrostatic pressure. 

The waters which deposit vein stones and ores are commonly 
hot, and in many cases they have derived their heat from intru- 
sions of igneous rock still uncooled within the crust. The sol- 
vent power of the water is thus greatly increased, and it takes 
up into solution various substances from the igneous and sedi- 
mentary rocks which it traverses. For various reasons these sub- 
stances are deposited in the vein as ores and vein stones. On 
rising through the fissure the water cools and loses pressure, and 
its capacity to hold minerals in solution is therefore lessened. 
Besides, as different currents meet in the fissure, some ascend- 
ing, some descending, and some coming in from the sides, the 
chemical reaction of these various weak solutions upon one 
another and upon the walls of the vein precipitates the minerals 
of vein stuffs and ores. 

As an illustration of the method of vein deposits we may cite the case 
of a wooden box pipe used in the Comstock mines, Nevada, to carry the 
hot water of the mine from one level to another, which in ten years 
was lined with calcium carbonate more than half an inch thick. 



METAMORPHISM AND MINERAL VEINS 289 

The Steamboat Springs, Nevada, furnish examples of mineral veins 
in process of formation. The steaming water rises through fissures in 
volcanic rocks and is now depositing in the rifts a vein stone of quartz, 
with metallic ores of iron, mercury, lead, and other metals. 

Reconcentration. Near the base of the zone of solution veins 
are often stored with exceptionally large and valuable ore 
deposits. This local enrichment of the vein is due to the recon- 
centration of its metalliferous ores. As the surface of the land 




FIG. 259. Reconcentration of Ores in Mineral Veins 

A, original vein ; B, same after reconcentration ; v, mineral vein ; s, sur- 
face of ground (dotted line, former surface of the ground) ; sp, spring; 
o, vein leached of ores by descending waters in zone of solution; 
TO, rich ore deposits reconcentrated from above ; n, unchanged portion 
of vein 

is slowly lowered by weathering and running water, the zone of 
solution is lowered at an equal rate and encroaches constantly 
on the zone of cementation. The minerals of veins are therefore 
constantly being dissolved along their upper portions and carried 
down the fissures by ground water to lower levels, where they 
are redeposited. 

Many of the richest ore deposits are thus due to successive 
concentrations : the ores were leached originally from the rocks 



290 THE ELEMENTS OF GEOLOGY 

to a large extent by laterally seeping waters ; they were concen- 
trated in the ore deposits of the vein chiefly by ascending cur- 
rents; they have been reconcentrated by descending waters in 
the way just mentioned. 

The original source of the metals. It is to the igneous rocks 
that we may look for the original source of the metals of veins. 
Lavas contain minute percentages of various metallic com- 
pounds, and no doubt this was the case also with the igneous 
rocks which formed the original earth crust. Intrusive masses 
of molten rock, as they cool and solidify, may segregate rich 
mineral deposits, such as magnetic iron ore. They give off vast 
quantities of steam and other gases, which carry out metallic 
ores in solution and deposit them in the surrounding strata. 
In fissures these ores are carried farther by hot ascending 
waters and laid down in mineral veins. By the erosion of the 
igneous rocks the metals have been distributed among sedi- 
mentary strata, and even the sea has taken into solution an 
appreciable amount of gold and other metals, but in this 
widely diffused condition they are wholly useless to man. 
The concentration which has made them available is due to 
the interaction of many agencies. Earth movements fracturing 
deeply the rocks of the crust, the intrusion of heated masses, 
the circulation of underground waters, have all cooperated in 
the concentration of the metals of mineral veins. 

While fissure veins are the most important of mineral veins, the 
latter term is applied also to any water way which has been filled by simi- 
lar deposits from solution. Thus, in soluble rocks, such as limestones, 
joints enlarged by percolating water are sometimes filled with metallif- 
erous deposits, as, for example, the lead and zinc deposits of the upper 
Mississippi valley. Even a porous aquifer may be made the seat of 
mineral deposits, as in the case of some copper-and-silver-bearing sand- 
stones of New Mexico. 




Key to Colors and Letters 

r ,Cai 

I I (W.of the Great Plains) C 

| | Devonian I) 

1 Silurian and 

J I Ordovk-iaii O 

Cambrian e 



f 1 Quaternary I 1 Carboniferous 

I I (W.of the Rocky Mts.)Q 

Tertiary T 
Cretaceous K 



rnrasalc and 

['i-iussu- J 



"i Peniisylvaiilan 

I \t ' 



land Permian P 



Pre-Cambrian A 



I I Mississlpplan M I I Igneous I 



O 



FIG. 260. Geological Map 



GULF OF MEXICO 




3d States and Part of Canada 



PAET III 

HISTORICAL GEOLOGY 

CHAPTER XIV 
THE GEOLOGICAL RECORD 

What a formation records. We have already learned that 
each individual body of stratified rock, or formation, constitutes 
a record of the time when it was laid. The structure and the 
character of the sediments of each formation tell whether the 
area was land or sea at the time when they were spread ; and 
if the former, whether the land was river plain, or lake bed, 
or was covered with wind-blown sands, or by the deposits of 
an ice sheet. If the sediments are marine, we may know also 
whether they were laid in shoal water near the shore or in 
deeper water out at sea, and whether during a period of emer- 
gence, or during a period of subsidence when the sea transgressed 
the land. By the same means each formation records the stage 
in the cycle of erosion of the land mass from which its sediments 
were derived (p. 185). An unconformity between two marine 
formations records the fact that between the periods when 
they were deposited in the sea the area emerged as land and 
suffered erosion (p. 227). The attitude and structure of the 
strata tell also of the foldings and fractures, the deformation 
and the metamorphism, which they have suffered; and the 
igneous rocks associated with them as lava flows and igneous 
intrusions add other details to the story. Each formation is 
thus a separate local chapter in the geological history of the 

291 



292 THE ELEMENTS OF GEOLOGY 

earth, and its strata are its leaves. It contains an authentic 
record of the physical conditions the geography of the 
time and place when and where its sediments were laid. 

Past cycles of erosion. These chapters in the history of the 
planet are very numerous, although much of the record has been 
destroyed in various ways. A succession of different formations 
is usually seen in any considerable section of the crust, such as 
a deep canyon or where the edges of upturned strata are exposed 
to view on the flanks of mountain ranges ; and in any extensive 
area, such as a state of the Union or a province of Canada, the 
number of formations outcropping on the surface is large. 

It is thus learned that our present continent is made up for 
the most part of old continental deltas. Some, recently emerged 
as the strata of young coastal plains, are the records of recent 
cycles of erosion ; while others were deposited in the early his- 
tory of the earth, and in many instances have been crumpled 
into mountains, which afterwards were leveled to their bases 
and lowered beneath the sea to receive a cover of later sedi- 
ments before they were again uplifted to form land. 

The cycle of erosion now in progress and recorded in the 
layers of stratified rock being spread beneath the sea in conti- 
nental deltas has therefore been preceded by many similar cycles. 
Again and again movements of the crust have brought to an 
end one cycle sometimes when only well under way, and 
sometimes when drawing toward its close and have begun 
another. Again and again they have added to the land areas 
which before were sea, with all their deposition records of 
earlier cycles, or have lowered areas of land beneath the sea to 
receive new sediments. 

The age of the earth. The thickness of the stratified rocks 
now exposed upon the eroded surface of the continents is very 
great. In the Appalachian region the strata are seven or eight 
miles thick, and still greater thicknesses have been measured in 
several other mountain ranges. The aggregate thickness of all 



THE GEOLOGICAL RECORD 293 

the formations of the stratified rocks of the earth's crust, giving 
to each formation its maximum thickness wherever found, 
amounts to not less than forty miles. Knowing how slowly 
sediments accumulate upon the sea floor (p. 184), we must 
believe that the successive cycles which the earth has seen 
stretch back into a past almost inconceivably remote, and 
measure tens of millions and perhaps even hundreds of millions 
of years. 

How the formations are correlated and the geological record 
made up. Arranged in the order of then- succession, the forma- 
tions of the earth's crust would constitute a connected record in 
which the geological history of the planet may be read, and 
therefore known as the geological record. But to arrange the 
formations in their natural order is not an easy task. A com- 
plete set of the volumes of the record is to be found in no single 
region. Their leaves and chapters are scattered over the land 
surface of the globe. In one area certain chapters may be 
found, though perhaps with many missing leaves, and with inter- 
vening chapters wanting, and these absent parts perhaps can be 
supplied only after long search through many other regions. 

Adjacent strata in any region are arranged according to the 
law of superposition, i.e. any stratum is younger than that on 
which it was deposited, just as in a pile of paper, any sheet was 
laid later than that on which it rests. Where rocks have been 
disturbed, their original attitude must be determined before the 
law can be applied. - Nor can the law of superposition be used 
in identifying and comparing the strata of different regions 
where the formations cannot be traced continuously from one 
region to the other. 

The formations of different regions are arranged in their true 
order by the law of included organisms ; i.e. formations, how- 
ever widely separated, which contain a similar assemblage of 
fossils are equivalent and belong to the same division of geo- 
logical time. 



294 THE ELEMENTS OF GEOLOGY 

The correlation of formations by means of fossils may be explained 
by the formations now being deposited about the north Atlantic. Litho- 
logically they are extremely various. On the continental shelf of North 
America limestones of different kinds are forming oft' Florida, and sand- 
stones and shales from Georgia northward. Separated from them by 
the deep Atlantic oozes are other sedimentary deposits now accumulat- 
ing along the west coast of Europe. If now all these offshore formations 
were raised to open air, how could they be correlated ? Surely not by 
lithological likeness, for in this respect they would be quite diverse. All 
would be similar, however, in the fossils which they contain. Some 
fossil species would be identical in all these formations and others 
would be closely allied. Making all due allowance for differences in 
species due to local differences in climate and other physical causes, it 
would still be plain that plants and animals so similar lived at the same 
period of time, and that the formations in which their remains were 
imbedded were contemporaneous in a broad way. The presence of the 
bones of whales and other marine mammals would prove that the strata 
were laid after the appearance of mammals upon earth, and imbedded 
relics of man would give a still closer approximation to their age. In 
the same way we correlate the earlier geological formations. 

For example, in 1902 there were collected the first fossils ever found 
on the antarctic continent. Among the dozen specimens obtained were 
some fossil ammonites (a family of chambered shells) of genera which 
are found on other continents in certain formations classified as the 
Cretaceous system, and which occur neither above these formations nor 
below them. On the basis of these few fossils we may be confident that 
the strata in which they were found in the antarctic region were laid 
in the same period of geologic time as were the Cretaceous rocks of the 
United States and Canada. 

The record as a time scale. By means of the law of included 
organisms and the law of superposition the formations of differ- 
ent countries and continents are correlated and arranged in 
their natural order. When the geological record is thus obtained 
it may be used as a universal time scale for geological history. 
Geological time is separated into divisions corresponding to the 
times during which the successive formations were laid. The 
largest assemblages of formations are known as groups/ while the 



THE GEOLOGICAL RECORD 295 

corresponding divisions of time are known as eras. Groups 
are subdivided into systems, and systems into series. Series 
are divided into stages and substages, subdivisions which 
do not concern us in this brief treatise. The corresponding 
divisions of time are given in the following table. 

Strata Time 

Group Era 

System Period 

Series Epoch 

The geologist is now prepared to read the physical history 
the geographical development of any country or of any conti- 
nent by means of its formations, when he has given each for- 
mation its true place in the geological record as a time scale. 

The following chart exhibits the main divisions of the record, 
the name given to each being given also to the corresponding 
time division. Thus we speak of the Cambrian system, mean- 
ing a certain succession of formations which are classified 
together because of broad resemblances in their included organ- 
isms ; and of the Cambrian period, meaning the time during 
which these rocks were deposited. 

Group and Era , System and Period Series and Epoch 

f Recent 
Quaternary .... j pleistocene 

Cenozoic .... * f Pliocene ' 

I Tertiary <j Miocene 

f- Cretaceous L Eocene 
Mesozoic . V . . J Jurassic 

L Triassic r Permian 

Carboniferous . . . < Pennsylvanian 

Devonian L Mississippian 



Paleozoic 



Silurian 



Ordovician 
^ Cambrian 
Proterozoic Era, Algonkian Group 
Archeozoic Era, Archean Group 



296 THE ELEMENTS OF GEOLOGY 



FOSSILS AND WHAT THEY TEACH 

The geological formations contain a record still more impor- 
tant than that of the geographical development of the conti- 
nents ; the fossils imbedded in the rocks of each formation tell 
of the kinds of animals and plants which inhabited the earth at 
that tune, and from these fossils we are therefore able to con- 
struct the history of life upon the earth. 

Fossils. These remains of organisms are found in the strata in all 
degrees of perfection, from trails and tracks and fragmentary impres- 
sions, to perfectly preserved shells, wood, bones, and complete skeletons. 
As a rule, it is only the hard parts of animals and plants which have 
left any traces in the rocks. Sometimes the original hard substance is 
preserved, but more often it has been replaced by some less soluble 
material. Petrifaction, as this process of slow replacement is called, is 
often carried on in the most exquisite detail. When wood, for example, 
is undergoing petrifaction, the woody tissue may be replaced, particle 
by particle, by silica in solution through the action of underground 
waters, even the microscopic structures of the wood being perfectly 
reproduced. In shells originally made of aragonite, a crystalline form 
of carbonate of lirne, that mineral is usually replaced by calcite, a more 
stable form of the same substance. The most common petrifying 
materials are calcite, silica, and pyrite (p. 13). 

Often the organic substance has neither been 
preserved nor replaced, but the form has been 
retained by means of molds and casts. Permanent 
impressions, or molds, may be made in sediments 
not only by the hard parts of organisms, but also 
FIG. 261. Section of by such soft and perishable parts as the leaves of 
Cast and Mold of p l an t s , and, in the rarest instances, by the skin of 
animals and the feathers of birds. In fine-grained 

a, shell; 6, mold of limestones even the imprints of jellyfish have been 

exterior; c, cast of . . , 

interior retained. 

The different kinds of molds and casts may be 

illustrated by means of a clam shell and some moist clay, the latter 
representing the sediments in which the remains of animals and plants 
are entombed. Imbedding the shell in the clay and allowing the clay 




THE GEOLOGICAL RECORD 297 

to harden, we have a mold of the exterior of the shell, as is seen on 
cutting the clay matrix in two and removing the shell from it. Filling 
this mold with clay of different color, we obtain a cast of the exterior, 
which represents accurately the original form and surface markings of 
the shell. In nature, shells and other relics of animals or plants are 
often removed by being dissolved by percolating waters, and the molds 
are either filled with sediments or with minerals deposited from 
solution. 

Where the fossil is hollow, a cast of the interior is made in the same 
way. Interior casts of shells reproduce any markings on the inside of 
the valves, and casts of the interior of the skulls of ancient vertebrates 
show the form and size of their brains. 

Imperfection of the life record. At the present time only the 
smallest fraction of the life on earth ever gets entombed in 
rocks now forming. In the forest great fallen tree trunks, as well 
as dead leaves, decay, and only add a little to the layer of dark 
vegetable mold from which they grew. The bones of land 
animals are, for the most part, left unburied on the surface and 
are soon destroyed by chemical agencies. Even where, as in the 
swamps of river flood plains and in other bogs, there are pre- 
served the remains of plants, and sometimes insects, together 
with the bones of some animal drowned or mired, in most cases 
these swamp and bog deposits are sooner or later destroyed by 
the shifting channels of the stream or by the general erosion 
of the land. 

In the sea the conditions for preservation are more favorable 
than on land; yet even here the proportion of animals and 
plants whose hard parts are fossilized is very small compared 
with those which either totally decay before they are buried in 
slowly accumulating sediments or are ground to powder by 
waves and currents. 

We may infer that during each period of the past, as at 
the present, only a very insignificant fraction of the innumer- 
able organisms of sea and land escaped destruction and left in 
continental and oceanic deposits permanent records of their 



298 THE ELEMENTS OF GEOLOGY 

existence. Scanty as these original life records must have 
been, they have been largely destroyed by metamorphism of 
the rocks in which they were imbedded, by solution in un- 
derground waters, and by the vast denudation under which 
the sediments of earlier periods have been eroded to furnish 
materials for the sedimentary records of later times. Moreover, 
very much of what has escaped destruction still remains undis- 
covered. The immense bulk of the stratified rocks is buried 
and inaccessible, and the records of the past which it contains 
can never be known. Comparatively few outcrops have been 
thoroughly searched for fossils. Although new species are con- 
stantly being discovered, each discovery may be considered as 
the outcome of a series of happy accidents, that the remains of 
individuals of this particular species happened to be imbedded 
and fossilized, that they happened to escape destruction during 
long ages, and that they happened to be exposed and found. 

Some inferences from the records of the history of life upon 
the planet. Meager as are these records, they set forth plainly 
some important truths which we will now briefly mention. 

1. Each series of the stratified rocks, except the very deepest, 
contains vestiges of life. Hence the earth was tenanted by living 
creatures for an uncalculated length of time before human his- 
tory began. 

2. Life on the earth has been ever changing. The youngest 
strata hold the remains of existing species of animals and 
plants and those of species and varieties 'closely allied to them. 
Strata somewhat older contain fewer existing species, and in 
strata of a still earlier, but by no means an ancient epoch, 
no existing species are to be found ; the species of that epoch 
and of previous epochs have vanished from the living world. 
During all geological time since life began on earth old species 
have constantly become extinct and with them the genera and 
families to which they belong, and other species, genera, and 
families have replaced them. The fossils of each formation 



THE GEOLOGICAL RECORD 299 

differ on the whole from those of every other. The assemblage 
of animals and plants (the fauna-flora) of each epoch differs 
from that of every other epoch. 

In many cases the extinction of a type has been gradual ; in 
other instances apparently abrupt. There is no evidence that 
any organism once become extinct has ever reappeared. The 
duration of a species in tune, or its "vertical range" through 
the strata, varies greatly. Some species are limited to a stratum 
a few feet in thickness; some may range through an entire 
formation and be found but little modified in still higher beds. 
A formation may thus often be divided into zones, each char- 
acterized by its own peculiar species. As a rule, the simpler 
organisms have a longer duration as species, though not as in- 
dividuals, than the more complex. 

3. The larger zoological and botanical groupings survive 
longer than the smaller. Species are so short-lived that a single 
geological epoch may be marked by several more or less com- 
plete extinctions of the species of its fauna-flora and their 
replacement by other species. A genus continues with new 
species after all the species with which it began have become 
extinct. Families survive genera, and orders families. Classes 
are so long-lived that most of those which are known from the 
earliest formations are represented by living forms, and no sub- 
kingdom has ever become extinct. 

Thus, to take an example from the stony corals, the Zoantharia, 
the particular characters which constituted a certain species Favo- 
sites niagarensis. of the order are confined to the Niagara series. Its 
generic characters appeared in other species earlier in the Silurian and 
continued through the Devonian. Its family characters, represented in 
different genera and species, range from the Ordovician to the close of 
the Paleozoic ; while the characters which it shares with all its order, the 
Zoantharia, began in the Cambrian and are found in living species. 

4. The change in organisms has been gradual. The fossils 
of each life zone and of each formation of a conformable series 



300 THE ELEMENTS OF GEOLOGY 

closely resemble, with some explainable exceptions, those of the 
beds immediately above and below. The animals and plants 
which tenanted the earth during any geological epoch are so 
closely related to those of the preceding and the succeeding 
epochs that we may consider them to be the descendants of the 
one and the ancestors of the other, thus accounting for the resem- 
blance by heredity. It is therefore believed that the species of 
animals and plants now living on the earth are the descendants 
of the species whose remains we find entombed in the rocks, and 
that the chain of life has been unbroken since its beginning. 

5. The change in species has been a gradual differentiation. 
Tracing the lines of descent of various animals and plants of 
the present backward through the divisions of geologic time, we 
find that these lines of descent converge and unite in simpler 
and still simpler types. The development of life may be repre- 
sented by a tree whose trunk is found in the earliest ages and 
whose branches spread and subdivide to the growing twigs of 
present species. 

6. The change in organisms throughout geologic time has been 
a progressive change. In the earliest ages the only animals and 
plants on the earth were lowly forms, simple and generalized in 
structure ; while succeeding ages have been characterized by 
the introduction of types more and more specialized and com- 
plex, and therefore of higher rank in the scale of being. Thus 
the Algonkian contains the remains of only the humblest forms 
of the invertebrates. In the Cambrian, Ordovician, and Silurian 
the invertebrates were represented in all their subkingdoms by 
a varied fauna. In the Devonian, fishes the lowest of the 
vertebrates became abundant. Amphibians made their entry 
on the stage in the Carboniferous, and reptiles came to rule the 
world in the Mesozoic. Mammals culminated in the Tertiary 
in strange forms which became more and more like those of the 
present as the long ages of that era rolled on ; and latest of all 
appeared the noblest product of the creative process, man. 



THE GEOLOGICAL RECORD 301 

Just as growth is characteristic of the individual life, so 
gradual, progressive change, or evolution, has characterized the 
history of life upon the planet. The evolution of the organic 
kingdom from its primitive germinal forms to the complex and 
highly organized fauna-flora of to-day may be compared to the 
growth of some noble oak as it rises from the acorn, spreading 
loftier and more widely extended branches as it grows. 

7. While higher and still higher types have continually been 
evolved, until man, the highest of all, appeared, the lower and 
earlier types have ge f fierally persisted. Some which reached 
their culmination early in the history of the earth have since 
changed only in slight adjustments to a changing environment. 
Thus the brachiopods, a type of shellfish, have made no prog- 
ress since the Paleozoic, and some of then- earliest known genera 
are represented by living forms hardly to be distinguished from 
their ancient ancestors. The lowest and earliest branches of the 
tree of life have risen to no higher levels since they reached 
their climax of development long ago. 

8. A strange parallel has been found to exist between the 
evolution of organisms and the development of the individual. 
In the embryonic stages of its growth the individual passes 
swiftly through the successive stages through which its ances- 
tors evolved during the millions of years of geologic time. 
The development of the individual recapitulates the evolution 
of the race. 

The frog is a typical amphibian. As a tadpole it passes through a 
stage identical in several well-known features with the maturity of 
fishes ; as, for example, its aquatic life, the tail by which it swims, and 
the gills through which it breathes. It is a fair inference that the tad- 
pole stage in the life history of the frog represents a stage in the evolu- 
tion of its kind, that the Amphibia are derived from fishlike ancestral 
forms. This inference is amply confirmed in the geological record; 
fishes appeared before Amphibia and were connected with them by 
transitional forms. 



302 THE ELEMENTS OF GEOLOGY 

The great length of geologic time inferred from the slow 
change of species. Life forms, like land forms, are thus subject 
to change under the influence of their changing environment and 
of forces acting from within. How slowly they change may be 
seen in the apparent stability of existing species. In the lifetime 
of the observer and even in the recorded history of man, species 
seem as stable as the mountain and the river. But life forms 
and land forms are alike variable, both in nature and still more 
under the shaping hand of man. As man has modified the face 
of the earth with his great engineering works, so he has pro- 
duced widely different varieties of many kinds of domesticated 
plants and animals, such as the varieties of the dog and the 
horse, the apple and the rose, which may be regarded in some 
respects as new species in the making. We have assumed that 
land forms have changed in the past under the influence of 
forces now in operation. Assuming also that life forms have 
always changed as they are changing at present, we come to 
realize something of the immensity of geologic time required 
for the evolution of life from its earliest lowly forms up to man. 

It is because the onward march of life has taken the same 
general course the world over that we are able to use it as a 
universal time scale and divide geologic time into ages and 
minor subdivisions according to the ruling or characteristic 
organisms then living on the earth. Thus, since vertebrates 
appeared, we have in succession the Age of Fishes, the Age of 
Amphibians, the Age of Reptiles, and the Age of Mammals. 

The chart given on page 295 is thus based 011 the law of 
superposition and the law of the evolution of organisms. The 
first law gives the succession of the formations in local areas. 
The fossils which they contain demonstrate the law of the pro- 
gressive appearance of organisms, and by means of this law the 
formations of different countries are correlated and set each in 
its place in a universal time scale and grouped together accord- 
ing to the affinities of their imbedded organic remains, 



THE GEOLOGICAL RECORD 303 

Geologic time divisions compared with those of human history. We 
may compare the division of geologic time into eras, periods, and other 
divisions according to the dominant life of the time, to the ill-defined 
ages into which human history is divided according to the dominance of 
some nation, ruler, or other characteristic feature. Thus we speak of the 
Dark Ages, the Age of Elizabeth, and the Age of Electricity. These crude 
divisions would be of much value if, as in the case of geologic time, we 
had no exact reckoning of human history by years. 

And as the course of human history has flowed in an unbroken 
stream along quiet reaches of slow change and through periods of rapid 
change and revolution, so with the course of geologic history. Periods 
of quiescence, in which revolutionary forces are perhaps gathering head, 
alternate with periods of comparatively rapid change in physical geog- 
raphy and' in organisms, when new and higher forms appear which serve 
to draw the boundary line of new epochs. Nevertheless, geological his- 
tory is a continuous progress; its periods and epochs shade into one 
another by imperceptible gradations, and all our subdivisions must needs 
be vague and more or less arbitrary. 

How fossils tell of the geography of the past. Fossils are 
used not only as a record of the development of life upon the 
earth, but also in testimony to the physical geography of past 
epochs. They indicate whether in any region the climate was 
tropical, temperate, or arctic. Since species spread slowly from 
some center of dispersion where they originate until some barrier 
limits their migration farther, the occurrence of the same species 
in rocks of the same system in different countries implies the 
absence of such barriers at the period. Thus in the collection 
of antarctic fossils referred to on page 294 there were shallow- 
water marine shells identical in species with Mesozoic shells 
found in India and in the southern extremity of South America. 
Since such organisms are not distributed by the currents of the 
deep sea and cannot migrate along its bottom, we infer a shal- 
low-water connection in Mesozoic times between India, South 
America, and the antarctic region. Such a shallow-water con- 
nection would be offered along the marginal shelf of a continent 
uniting these now widely separated countries. 



CHAPTER XV 
THE PRE-CAMBRIAN SYSTEMS 

The earth's beginnings. The geological record does not tell 
us of the beginnings of the earth. The history of the planet, 
as we have every reason to believe, stretches far back beyond 
the period of the oldest stratified rocks, and is involved in the 
history of the solar system. 

The nebular hypothesis. It was long held that the earth began as a 
vaporous shining sphere formed by the gathering together of the 
material of a gaseous ring which had been detached from a cooling 
and shrinking nebula. Such a vaporous sphere would condense to a 
liquid fiery globe whose surface would become cold and solid, while 
the interior would long remain intensely hot because of the slow con- 
ductivity of the crust. Under these conditions the primeval atmosphere 
of the earth must have contained in vapor the water now belonging to 
the earth's crust and surface. It also held all the oxygen since locked 
up in rocks by their oxidation, and all the carbon dioxide which has since 
been laid away in limestones, besides that corresponding to the carbon 
of carbonaceous deposits, such as peat, coal, and petroleum. On this 
hypothesis the original atmosphere was dense, dark, and noxious, and 
enormously heavier than the atmosphere at present. Strong objections 
have been raised to the nebular hypothesis, and it has been abandoned 
by many authorities. 

The planetesimal hypothesis. Nebulae of the spheroidal type, such as 
the nebular hypothesis requires, are rare ; but another type, the spiral, 
is very common. A spiral nebula consists of a central luminous mass 
from which there extend from opposite sides two arms more or less 
closely coiled. Each arm shows luminous knots irregularly placed. 

Such a spiral nebula might be formed by the close approach of one 
star to another, of a passing star to our own sun, for example, before 
the birth of the solar system. As the pull of the rnoon raises the tides on 
opposite sides of the earth, so, it is supposed, the pull of the passing 

304 



THE PRE-CAMBRIAN SYSTEMS 305 

star released the explosive forces of the sun, and two streams of mat- 
ter were flung out from it into space, one toward the star and the other 
directly away from it. The knots in these arms formed the nuclei of 
the planets. The gaseous matter scattered outside the knots cooled into 
small stony masses, revolving about the central mass and hence called 
planetesimals (little planets). Like the meteorites which still fall upon 
the earth, the planetesimals were gradually gathered in by the nuclear 
knots, which thus grew to the present planets. 

Such cold, stony bodies might have come together at so slow a rate 
that the heat caused by their impact would not raise sensibly the tem- 
perature of the growing planet. Thus the surface of the earth may 
never have been hot and luminous ; but as the loose aggregation of 
stony masses grew larger and was more and more compressed by its 
own gravitation, the heat thus generated raised the interior to high 
temperatures, while from time to time molten rock was intruded among 
the loose, cold meteoritic masses of the crust and outpoured upon the 
surface. 

It is supposed that the meteorites of which the earth was built 
brought to it, as meteorites do now, various gases shut up within their 
pores. As the heat of the interior increased, these gases transpired to 
the surface and formed the primitive atmosphere and hydrosphere. 
The atmosphere has therefore grown slowly from the smallest begin- 
nings. Gases emitted from th'e interior in volcanic eruptions and in 
other ways have ever added to it, and are adding to it now. On the 
other hand, the atmosphere has constantly suffered loss, as it has been 
robbed of oxygen by the oxidation of rocks in weathering, and of 
carbon dioxide in the making of limestones and carbonaceous deposits. 

While all hypotheses of the earth's beginnings are as yet 
unproved speculations, they serve to bring to mind one of the 
chief lessons which geology has to teach, that the duration of 
the earth in time, like the extension of the universe in space, is 
vastly beyond the power of the human mind to realize. 

We pass now from the dim realm of speculation to the earli- 
est era of the recorded history of the earth, where some certain 
facts may be observed and some sure inferences from them 
may be drawn. 



306 THE ELEMENTS OF GEOLOGY 

ARCHEOZOIC ERA (Greek, arc-he, beginning; zoe, life): 
ARC HE AN GROUP 

The oldest rocks of the earth's crust form an intricate com- 
plex of sedimentary and volcanic rocks, crushed, crumpled, and 
metamorphosed, and injected with dikes, bosses, and other 
igneous intrusions. The base of the complex is nowhere to be 
seen, so that the original surface of the earth's crust on which 
the first sediments were laid is quite unknown. So thick are 
the sediments of this group that the Archeozoic era may well 
have been longer than all later geologic time. Thus, east of 
Lake Huron there outcrop the upturned edges of a series of 
metamorphosed limestone and other sedimentary rocks, whose 
total thickness is nearly eighteen miles, the deepest pile of 
lime rock on the earth. 

The well-nigh universal crumpling and metamorphism of the 
Archean group, and the immense volume of its igneous intru- 
sions, prove that the long cycle of quiet sedimentation was 
closed by a period of the intensest deformation. Vast quantities 
of molten rock rose from the hot 'depths of the earth, fus- 
ing the basal portions of the Archean sediments and raising 
them into mountains with granitic cores, while the strata were 
mashed, broken, folded, and altered on a scale never afterwards 
approached. 

The contacts of the gnarled Archean rocks with the overlying 
sediments show that before the opening of the next era the 
Archeozoic mountains had been worn down to their roots. 



PROTEROZOIC ERA: ALGOXKIAN GROUP 

In some regions there rests unconformably on the Archean 
an immense body of stratified rocks, thousands and in places 
even scores of thousands of feet thick, known as the Algonkian. 



THE PRE-CAMBRIAN SYSTEMS 307 

Great unconformities divide it into well-defined systems, but as 
only the scantiest traces of fossils appear here and there among 
its strata, it is as yet impossible to correlate the formations of 
different regions and to give them names of more than local 
application. We will describe the Algonkian rocks of two typi- 
cal areas. 

The Grand Canyon of the Colorado. We have already studied 
a very ancient peneplain whose edge is exposed to view deep on 
the walls of the Colorado Canyon (nn 1 , Fig. 207). The formation 
of flat-lying sandstone which covers this buried land surface is 
proved by its fossils to belong to the Cambrian, the earliest 
period of the Paleozoic era. The tilted rocks (b, Fig. 207) on 
whose upturned edges the Cambrian sandstone rests are far 
older, for the physical break which separates them from it 
records a tune interval during which they were upheaved to 
mountainous ridges and worn down to a low plain. They are 
therefore classified as Algonkian. They comprise two immense 
series. The upper is more than five thousand feet thick and 
consists of shales and sandstones with some limestones. Sepa- 
rated from it by an unconformity which does not appear in 
Figure 207, the lower division, seven thousand feet thick, con- 
sists chiefly of massive reddish sandstones with seven or more 
sheets of lava interbedded. The lowest member is a basal con- 
glomerate composed of pebbles derived from the erosion of the 
dark crumpled schists beneath, schists which are supposed to 
be Archean. As shown in Figure 207, a strong unconformity 
(mm 1 , Fig. 207) parts the schists and the Algonkiau. The floor 
on which the Algonkian rests is remarkably even, and here 
again is proved an interval of incalculable length, during which 
an ancient land mass of Archean rocks was baseleveled before 
it received the cover of the sediments of the later age. 

The Lake Superior region. In eastern Canada an area of 
pre-Cambrian rocks, Archean and Algonkian, estimated at two 
million square miles, stretches from the Great Lakes and the 



308 



THE ELEMENTS OF GEOLOGY 



s 

I 

X 



St. Lawrence River northward to the confines of 
the continent, inclosing Hudson Bay in the arms 
of a gigantic U. This immense area, which we 
have already studied as the Laurentian peneplain 
(p. 89), extends southward across the Canadian 
border into northern Minnesota, Wisconsin, and 
Michigan. The rocks of this area are known to 
be pre-Cambrian ; for the Cambrian strata, wher- 
ever found, lie unconformably upon them. 

The general relations of the formations of that 
portion of the area which lies about Lake Supe- 
rior are shown in Figure 262. Great unconformi- 
ties, UU f , separate the Algonkiaii both from 
the Archean and from the Cambrian, and divide 
it into three distinct systems, the Lower Hu- 
ronian, the Upper Huronian, and the Kewee- 
nawan. The Lower and the Upper Huronian 
consist in the main of old sea muds and sands 
and limy oozes now changed to gneisses, schists, 
marbles, quartzites, slates, and other metamorphic 
rocks. The Keweenawan is composed of immense 
piles of lava, such as those of Iceland, overlain by 
bedded sandstones. What remains of these rock 
systems after the denudation of all later geo- 
logic ages is enormous. The Lower Huronian is 
more than a mile thick, the Upper Huronian more 
than two miles thick, while the Keweenawan ex- 
ceeds nine miles in thickness. The vast length 
of Algonkian time is shown by the thickness 
of its marine deposits and by the cycles of ero- 
sion which it includes. In Figure 262 the stu- 
dent may read an outline of the history of the 
Lake Superior region, the deformations which 
it- suffered, their relative severity, the times 



THE PRE-CAMBRIAN SYSTEMS 309 

when they occurred, and the erosion cycles marked by the 
successive unconformities. 

Other pre-Cambrian areas in North America. Pre-Cambrian 
rocks are exposed in various parts of the continent, usually by 
the erosion of mountain ranges in which their strata were 
infolded. Large areas occur in the maritime provinces of 
Canada. The core of the Green Mountains of Vermont is pre- 
Cambrian, and rocks of these systems occur in scattered patches 
in western Massachusetts. Here belong also the oldest rocks of 
the Highlands of the Hudson and of New Jersey. The Adi- 
rondack region, an outlier of the Laurentian region, exposes 
pre-Cambrian rocks, which have been metamorphosed and tilted 
by the intrusion of a great boss of igneous rock out of which 
the central peaks are carved. The core of the Blue Eidge and 
probably much of the Piedmont Belt are of this age. In the 
Black Hills the irruption of an immense mass of granite has 
caused or accompanied the upheaval of pre-Cambrian strata and 
metamorphosed them by heat and pressure into gneisses, schists, 
quartzites, and slates. In most of these mountainous regions 
the lowest strata are profoundly changed by metamorphism, and 
they can be assigned to the pre-Cambrian only where they are 
clearly overlain unconformably by formations proved to be Cam- 
brian by their fossils. In the Belt Mountains of Montana, how- 
ever, the Cambrian is underlain by Algonkian sediments twelve 
thousand feet thick, and but little altered. 

Mineral wealth of .the pre-Cambrian rocks. The pre-Cam- 
brian rocks are of very great economic importance, because of 
their extensive metamorphism and the enormous masses of igne- 
ous rock which they involve. In many parts of the country 
they are the source of supply of granite, gneiss, marble, slate, 
and other such building materials. Still more valuable are the 
stores of iron and copper and other metals which they contain. 

At the present tune the pre-Cambrian region about Lake 
Superior leads the world in the production of iron ore, its 



310 THE ELEMENTS OF GEOLOGY 

output for 1903 being more than five sevenths of the entire 
output of the whole United States, and exceeding that of any 
foreign country. The ore bodies consist chiefly of the red 
oxide of iron (hematite) and occur in troughs of the strata, 
underlain by some impervious rock. A theory held by many 
refers the ultimate source of the iron to the igneous rocks of 
the Archean. When these rocks were upheaved and subjected 
to weathering, their iron compounds were decomposed. Their 
iron was leached out and carried away to be laid in the Algon- 
kian water bodies in beds of iron carbonate and other iron 
compounds. During the later ages, after the Algonkian strata 
had been uplifted to form part of the continent, a second con- 
centration has taken place. Descending underground waters 
charged with oxygen have decomposed the iron carbonate and 
deposited the iron, in the form of iron oxide, in troughs 
of the strata where their downward progress was arrested by 
impervious floors. 

The pre-Cambrian rocks of the eastern United States also are 
rich in iron. In certain districts, as in the Highlands of New 
Jersey, the black oxide of iron (magnetite) is so abundant in 
beds and disseminated grains that the ordinary surveyor's com- 
pass is useless. 

The pre-Cambrian copper mines of the Lake Superior region 
are among the richest on the globe. In the igneous rocks copper, 
next to iron, is the most common of all the useful metals, and 
it was especially abundant in the Keweenawan lavas. After 
the Keweenawan was uplifted to form land, percolating waters 
leached out much of the copper diffused in the lava sheets and 
deposited it within steam blebs as amygdules of native copper, 
in cracks and fissures, and especially as a cement, or matrix, in 
the interbedded gravels which formed the chief aquifers of the 
region. The famous Calumet and Hecla mine follows down the 
dip of the strata to the depth of more than a mile and works 
such a conglomerate whose matrix is in part pure copper. 



THE PRE-CAMBRIAN SYSTEMS 



311 



The appearance of life. It was probably during the Archeo- 
zoic that life appeared on earth. Creative energy was mani- 
fested upon a higher plane than that of the physical and 
chemical reactions which alone had made earth's history so far. 
Geology can offer no evidence as to whence or how life came, 
but it has much to reveal of the marvelous development of 
the forms of life during the geologic ages. Its earliest forms 
are unknown, but analogy suggests 
that as every living creature has 
developed from a single cell, so the 
earliest organisms upon the globe 

the germs from which all later 
life is supposed to have been evolved 

were tiny, unicellular masses of 
protoplasm, resembling the amoeba 
of to-day in the simplicity of their 
structure. 

Such lowly forms were destitute 
of any hard parts and could leave 
no evidence of their existence in 
the record of the rocks. And of 
their supposed descendants we find 
so few traces in the pre-Cambrian 
strata that the first steps in organic F IG . 263. Successive Stages in 
evolution must be Supplied from the Development of the Ovum 
such analogies in embryology as the to the Gastrula Stage 
following. The fertilized ovum, the cell with which each ani- 
mal begins its life, grows and multiplies by cell division, and 
develops into a hollow globe of cells called the blastosphere. 
This stage is succeeded by the stage of the gastrula, an ovoid 
or cup-shaped body with a double wall of cells inclosing a 
body cavity, and with an opening, the primitive mouth. Each of 
these early embryological stages is represented by living ani- 
mals, the undivided cell by the protozoa, the blastosphere by 




312 THE ELEMENTS OF GEOLOGY 

some rare forms, and the gastrula in the essential structure 
of the ccdenterates, the subkingdom to which the fresh-water 
hydra and the corals belong. All forms of animal life, from 
the ccelenterates to the mammals, follow the same path in their 
embryological development as far as the gastrula stage, but 
here their paths widely diverge, those of each subkingdom going 
their own separate ways. 

We may infer, therefore, that during the pre-Cambrian periods 
organic evolution followed the lines thus dimly traced. The 
earliest one-celled protozoa were probably succeeded by many- 
celled animals of the type of the blastosphere, and these by 
gastrula-like organisms. From the gastrula type the higher sub- 
divisions of animal life probably diverged, as separate branches 
from a common trunk. Much or all of this vast differentia- 
tion was accomplished before the opening of the next era ; for 
all the subkingdoms are represented in the Cambrian except 
the vertebrates. 

Evidences of pre-Cambrian life. An indirect evidence of life 
during the pre-Cambrian periods is found in the abundant and 
varied fauna of the next period ; for, if the theory of evolution 
is correct, the differentiation of the Cambrian fauna was a long 
process which might well have required for its accomplishment 
a large part of pre-Cambrian time. 

Other indirect evidences are the pre-Cambrian limestones, 
iron ores, and graphite deposits, since such minerals and rocks 
have been formed in later times by the help of organisms. If the 
carbonate of lime of the Algonkian limestones and marbles was 
extracted from sea water by organisms, as is done at present by 
corals, mollusks, and other humble animals and plants, the 
life of those ancient seas must have been abundant. Graphite, 
a soft black mineral composed of carbon and used in the man- 
ufacture of lead pencils and as a lubricant, occurs widely in 
the metamorphic pre-Cambrian rocks. It is known to be pro- 
duced in some cases by the metamorphism of coal, which itself 



THE PRE-CAMBRIAN SYSTEMS 313 

is formed of decomposed vegetal tissues. Seams of graphite 
may therefore represent accumulations of vegetal matter such 
as seaweed. But limestone, iron ores, and graphite can be pro- 
duced by chemical processes, and their presence in the pre- 
Cambrian makes it only probable, and not certain, that life 
existed at that time. 

Pre-Cambrian fossils. Very rarely has any clear trace of an 
organism been found in the most ancient chapters of the geo- 
logical record, so many of their leaves have been destroyed and so 
far have their pages been defaced. Microscopic radiolaria and 
bacteria have been found. Burrows and trails of several species 
of annelid worms appear in the sandstones of ancient beaches. 
Fragments have been reported which may belong to brachiopods 
and large crustaceans. And in the limestones of the Proterozoic 
occur reefs of coral-like calcareous algae whose heads are some- 
times two feet thick. These diverse forms indicate that before 
the Algonkian had closed, life was abundant and had widely 
differentiated. We may expect that other forms will be dis- 
covered as the rocks are closely searched. 

Pre-Cambrian geography. Our knowledge is far too meager 
to warrant an attempt to draw the varying outlines of sea and 
land during the Archean and Algonkian eras. Pre-Cambrian 
time probably was longer than all later geological time down 
to the present, as we may infer from the vast thicknesses of its 
rocks and the unconformities which part them. We know that 
during its long periods land masses again and again rose from 
the sea, were worn low, and were submerged and covered with 
the waste of other lands. But the formations of separated 
regions cannot be correlated because of the absence of fossils, 
and nothing more can be made out than the detached chapters 
of local histories, such as the outline given of the district about 
Lake Superior. 

The pre-Cambrian rocks show no evidence of any forces then 
at work upon the earth except the forces which are at work 



314 THE ELEMENTS OF GEOLOGY 

upon it now. The most ancient sediments known are so like 
the sediments now being laid that we may infer that they were 
formed under conditions essentially similar to those of the 
present time. There is no proof that the sands of the pre- 
Cambrian sandstones were swept by any more powerful waves 
and currents than are offshore sands to-day, or that the muds 
of the pre-Cambrian shales settled to the sea floor in less quiet 
water than such muds settle in at present. The pre-Cambrian 
lands were, no doubt, worn by wind and weather, beaten by 
rain, and furrowed by streams as now, and, as now, they 
fronted the ocean with beaches on which waves dashed and 
along which tidal currents ran. They w r ere also scoured by 
continental ice sheets during recurring glacial epochs, for amid 
the Proterozoic rocks widespread ground moraines are found 
in Canada, India, South Africa, and Australia. 

Perhaps the chief difference between the pre-Cambrian and 
the present was the absence of life upon the land. So far as we 
have any knowledge, no forests covered the mountain sides, no 
verdure carpeted the plains, and no animals lived on the 
ground or in the air. It is permitted to think of the most 
ancient lands as deserts of barren rock and rock waste swept 
by rains and trenched by powerful streams. We may there- 
fore suppose that the processes of their destruction went on 
more rapidly than at present. 



CHAPTEE XVI 
THE CAMBRIAN 

The Paleozoic era. The second volume of the geological 
record, called the Paleozoic (Greek, palaios, ancient ; zoe, life), 
has come down to us far less mutilated and defaced than has 
the first volume, which contains the traces of the most ancient 
life of the globe. Fossils are far more abundant in the Paleo- 
zoic than in the earlier strata, while the sediments in which 
they were entombed have suffered far less from metamorphism 
and other causes, and have been less widely buried from view, 
than the strata of the pre-Cambrian groups. By means of their 
fossils we can correlate the formations of widely separated 
regions from the beginning of the Paleozoic on, and can there- 
fore trace some outline of the history of the continents. 

Paleozoic time, although shorter than the pre-Cambrian as 
measured by the thickness of the strata, must still be reckoned 
in millions of years. During this vast reach of time the changes 
in organisms were very great. It is according to the successive 
stages in the advance of life that the Paleozoic formations are 
arranged in five systems, the Cambrian, the Ordovician, the 
Silurian, the Devonian, and the Carboniferous. On the same 
basis the first three systems are grouped together as the older 
Paleozoic, because they alike are characterized by the dominance 
of the invertebrates ; while the last two systems are united in 
the later Paleozoic, and are characterized, the one by the domi- 
nance of fishes, and the other by the appearance of amphibians 
and reptiles. 

Each of these systems is world-wide in its distribution, and 
may be recognized on any continent by its own peculiar fauna. 

315 



316 THE ELEMENTS OF GEOLOGY 

The names first given them in Great Britain have therefore 
come into general use, while their subdivisions, which often 
cannot be correlated in different countries and different regions, 
are usually given local names. 

The first three systems were named from the fact that their strata 
are well displayed in Wales. The Cambrian carries the Roman name 
of Wales, and the Ordovician and Silurian the names of tribes of 
ancient Britons which inhabited the same country. The Devonian is 
named from the English county Devon, where its rocks were early 
studied. The Carboniferous was so called from the large amount of 
coal which it was found to contain in Great Britain and continental 
Europe. 

THE CAMBRIAN 

Distribution of strata. The Cambrian rocks outcrop in narrow 
belts about the pre-Cambrian areas of eastern Canada and the 
Lake Superior region, the Adirondacks and the Green Mountains. 
Strips of Cambrian formations occupy troughs in the pre-Cam- 
brian rocks of New England and the maritime provinces of 
Canada ; a long belt borders on the west the crystalline rocks of 
the Blue Ridge ; and on the opposite side of the continent the 
Cambrian reappears in the mountains of the Great Basin and the 
Canadian Rockies. In the Mississippi valley it is exposed in 
small districts where uplift has permitted the stripping off' of 
younger rocks. Although the areas of outcrop are small, we 
may infer that Cambrian rocks were widely deposited over the 
continent of North America. 

Physical geography. The Cambrian system of North Amer- 
ica comprises three distinct series, the Lower Cambrian, the 
Middle Cambrian, and the Upper Cambrian, each of which is 
characterized by its own peculiar fauna. In sketching the out- 
lines of the continent as it was at the beginning of the Paleozoic, 
it must be remembered that wherever the Lower Cambrian 
formations now are found was certainly then sea bottom, and 



THE CAMBRIAN 



317 



wherever the. Lower Cambrian are wanting, and the next forma- 
tions rest directly on pre-Cambrian rocks, was probably then land. 

Early Cambrian geography. In this way we know that at 
the opening of the Cambrian two long, narrow mediterranean 
seas stretched from north to south across the continent. The 
eastern sea extended from the Gulf of St. Lawrence down the 
Champlain-Hudson 
valley and thence 
along the western 
base of the Blue 
Eidge south at 
least to Alabama. 
The western sea 
stretched from the 
Canadian Rockies 
over the Great 
Basin and at least 
as far south as the 
Grand Canyon of 
the Colorado in 
Arizona. 

Between these 
mediterraneans lay 
a great central land 
which included the 
pre-Cambrian U- 
shaped area of the 
Laurentian peneplain, and probably extended southward to the 
latitude of New Orleans. To the east lay a land which we may 
designate as Appalachia, whose western shore line was drawn 
along the site of the present Blue Ridge, but whose other limits 
are quite unknown. The land of Appalachia must have been 
large, for it furnished a great amount of waste during the entire 
Paleozoic era, and its eastern coast may possibly have lain even 




FIG. 264. Hypothetical Map of Eastern North 
America at the Beginning of Cambrian Time 

Unshaded areas, probable land 



318 THE ELEMENTS OF GEOLOGY 

beyond the edge of the present continental shelf. On the west- 
ern side of the continent a narrow land occupied the site of the 
Sierra Nevada Mountains. 

Thus, even at the beginning of the Paleozoic, the continental 
plateau of North America had already been left by crustal move- 
ments in relief above the abysses of the great oceans on either 
side. The mediterraneans which lay upon it were shallow, as 
their sediments prove. They were epicontinental seas; that is, 
they rested upon (Greek, epi) the submerged portion of the con- 
tinental plateau. We have no proof that the deep ocean ever 
occupied any part of where North America now is. 

The Middle and Upper Cambrian strata are found together 
with the Lower Cambrian over the area of both the eastern 
and the western mediterraneans, so that here the sea contin- 
ued during the entire period. The sediments throughout are 
those of shoal water. Coarse cross-bedded sandstones record 
the action of strong shifting currents which spread coarse waste 
near shore and winnowed it of finer stuff. Frequent ripple 
marks on the bedding planes of the strata prove that the loose 
sands of the sea floor were near enough to the surface to be 
agitated by waves and tidal currents. Sun cracks show that 
often the outgoing tide exposed large muddy flats to the drying 
action of the sun. The fossils, also, of the strata are of kinds 
related to those which now live in shallow waters near the 
shore. 

The sediments which gathered in the mediterranean seas 
were very thick, reaching in places the enormous depth of ten 
thousand feet. Hence the bottoms of these seas were sinking 
troughs, ever filling with waste from the adjacent land as fast 
as they subsided. 

Late Cambrian geography. The formations of the Middle 
and Upper Cambrian are found resting unconformably on the 
pre-Cambrian rocks from New York westward into Minnesota 
and at various points in the interior, as in Missouri and in 



THE CAMBRIAN 319 

Texas. Hence after earlier Cambrian time the central land sub- 
sided, with much the same effect as if the Mississippi valley 
were now to lower gradually, and the Gulf of Mexico to spread 
northward until it entered Lake Superior. The Cambrian seas 
transgressed the central land and strewed far and wide behind 
their advancing beaches the sediments of the later Cambrian 
upon an eroded surface of pre-Cambrian rocks. 

The succession of the Cambrian formations in North America 
records many minor oscillations and varying conditions of 
physical geography ; yet on the whole it tells of widening seas 
and lowering lands. Basal conglomerates and coarse sandstones 
which must have been laid near shore are succeeded by shaly 
sandstones, sandy shales, and shales. Toward the top of the 
series heavy beds of limestone, extending from the Blue Ridge 
to Missouri, speak of clear water, and either of more distant 
shores or of neighboring lands which were worn or sunk so 
low that for the most part their waste was carried to the sea 
in solution. 

In brief, the Cambrian was a period of submergence. It 
began with the larger part of North America emerged as great 
land masses. It closed with most of the interior of the con- 
tinental plateau covered with a shallow sea. 

THE LIFE OF THE CAMBRIAN PERIOD 

It is now for the first tune that we find preserved in the 
offshore deposits of the Cambrian seas enough remains of ani- 
mal life to be properly called a fauna. Doubtless these remains 
are only the most fragmentary representation of the life of the 
time, for the Cambrian rocks are very old and have been widely 
metamorphosed. Yet the five hundred and more species already 
discovered embrace all the leading types of invertebrate life, and 
are so varied that we must believe that their lines of descent 
stretch far back into the pre-Cambrian past. 



320 



THE ELEMENTS OF GEOLOGY 



Plants. No remains of plants have been found in Cambrian 
strata, except some doubtful markings, as of seaweed. 

Sponges. The sponges, the lowest 
of the multicellular animals, were 
represented by several orders. Their 
fossils are recognized by the sili- 
ceous spicules, which, as in modern 
sponges, either were scattered 
through a mass of horny fibers or 
were connected in a flinty frame- 
work. 

F.o.266. sponge Spicules as Coelenterates. This subkingdom 
seen in Flint under the includes two classes of interest to 
Microscope the geologist, the Hydrozoa, such 

as the fresh-water hydra and the jellyfish, and the corals. Both 
classes existed in the Cambrian. 

The Hydrozoa were represented not only by jellyfish but 
also by the graptolite, which takes its name from a fancied 






FIG. 200. Graptolites, Ordovician and Silurian Species 



THE CAMBRIAN 



321 



resemblance of 'some of its forms to a quill pen. It was a com- 
posite animal with a horny framework, the individuals of the 
colony living in cells strung on one or both sides along a hollow 
stem, and communicating by means of a common flesh in this 
central tube. Some graptolites were straight, and some curved 
or spiral; some were single stemmed, and others consisted of 
several radial stems united. Graptolites occur but rarely in 
the Upper Cambrian. In the Ordovician and Silurian they are 
very plentiful, and at 
the close of the Silurian 
they pass out of exist- 
ence, never to return. 

Corals are very rarely 
found in the Cambrian, 
and the description of 
their primitive types is 
postponed to later chap- 
ters treating of periods 
when they became more 
numerous. 

Echinoderms. This 
subkingdom comprises 
at present such familiar 
forms as the crinoid, the 
starfish, and the sea urchin. The structure of echinoderms is 
radiate. Their integument is hardened with plates or particles 
of carbonate of lime. 

Of the free echinoderms, such as the starfish and the sea urchin, 
the former 'has been found in the Cambrian rocks of Europe, 
but neither have so far been discovered in the strata of this 
period in North America, The stemmed and lower division of 
the echinoderms was represented by a primitive type, the 
cystoid, so called from its saclike form. A small globular or 
ovate "calyx" of calcareous plates, with an aperture at the 




FIG. 267. Cystoids, one showing Two Rudi- 
mentary Arms 



322 



THE ELEMENTS OF GEOLOGY 



top for the mouth, inclosed the body of the animal, and was 
attached to the sea bottom by a short flexible stalk consisting 
of disks of carbonate of lime held together by a central ligament. 
Arthropods. These segmented animals with " jointed feet," 
as their name suggests, may be divided in a general way into 
water breathers and air breathers. The first-named and lower 
division comprises the class of the Crustacea, arthropods 
protected by a hard exterior skeleton, or " crust," of which 
crabs, crayfish, and lobsters are familiar examples. The higher 




FIG. 268. Trilobites 

A, a Cambrian species; B, a Devonian species, showing eye; C, restoration 
of an Ordovician species 

division, that of the air breathers, includes the following classes : 
spiders, scorpions, centipedes, and insects. 

The trilobite. The aquatic arthropods, the Crustacea, culmi- 
nated before the air breathers ; and while none of the latter are 
found in the Cambrian, the former were the dominant life of 
the time in numbers, in size, and in the variety of their forms. 
The leading crustacean type is the trilobite, which takes its 
name from the three lobes into which its shell is divided longi- 
tudinally. There are also three cross divisions, the head shield, 



THE CAMBRIAN 323 

the tail shield, 'and between the two the thorax, consisting of a 
number of distinct and unconsolidated segments. The head 
shield carries a pair of large, crescentic, compound eyes, like 
those of the insect. The eye varies greatly in the number of its 
lenses, ranging from fourteen in some species to fifteen thousand 
in others. Figure 268, (7, is a restoration of the trilobite, and 
shows the appendages, which are found preserved only in the 
rarest cases. 

During the long ages of the Cambrian the trilobite varied 
greatly. Again and again new species and genera appeared, 
while the older types became extinct. For this reason and 
because of their abundance, trilobites are used in the classifica- 
tion of the Cambrian system. The Lower Cambrian is charac- 
terized by the presence of a trilobitic fauna in which the genus 
Olenellus is predominant. This, the Ole- 
nellus Zone, is one of the most important 
platforms in the entire geological series ; 
for, the w T orld over, it marks the begin- 
ning of Paleozoic time, while all under- 
lying strata are classified as pre-Cam- 

. , FIG. 269. APhyllopod 
brian. The Middle Cambrian is marked 

by the genus Paradoxides, and the Upper Cambrian by the 
genus Olenus. Some of the Cambrian trilobites were giants, 
measuring as much as two feet long, while others were the 
smallest of their kind, a fraction of an inch in length. 

Another type of crustacean which lived in the Cambrian and 
whose order is still living is illustrated in Figure 269. 

Worms. Trails and burrows of worms have been left on 
the sea beaches and mud flats of all geological times from the 
Algonkian to the present. 

Brachiopods. These soft-bodied animals, with bivalve shells 
and two interior armlike processes which served for breathing, 
appeared in the Algonkian, and had now become very abundant. 
The two valves of the brachiopod shell are unequal in size, and 




324 



THE ELEMENTS OF GEOLOGY 




in each valve a line drawn from the beak to the base divides 
the valve into two equal parts (Fig. 270). It may thus be told 

from the pelecypod mollusk, such as 
the clam, whose two valves are not far 
from equal in size, each being divided 
into unequal parts by a line dropped 
from the beak (Fig. 272). 
FIG. 270. A Cambrian Articu- Brachiopods include two orders. In 
late Brachiopod, Orthis the mogt pr i m i t i ve or d er that of the 

inarticulate brachiopods the two valves are held together 

only by muscles of the animal, and the shell is horny or is 

composed of phosphate of lime. The Discina, which began in 

the Algonkian, is of this type, as is 

also the Lingulella of the Cambrian 

(Fig. 271). Both of these genera 

have lived on during the millions 

of years of geological time since 

their introduction, handing down FIG. 271. Cambrian 





late Brachiopods 
A, Lingulella ; B, Discina 



from generation to generation with 
hardly any change to their descend- 
ants now living off our shores the characters impressed upon 
them at the beginning. 

The more highly organized articulate brachiopods have 
valves of carbonate of lime more securely joined by a hinge 
with teeth and sockets (Fig. 270). In the 
Cambrian the inarticulates predominate, 
though the articulates grow common 
toward the end of the period.. 

Mollusks. The three chief classes of 
mollusks the pelecypods (represented by 
the oyster and clam of to-day), the gastro- 
pods (represented now by snails, conches, and periwinkles), and 
the ceplialopods (such as the nautilus, cuttlefish, and squids) 
were all represented in the Cambrian, although very sparingly. 




FiG. 272. A Cambrian 
Pelecypod 



THE CAMBRIAN 



325 



Pteropods, a suborder of the gastropods, appeared in this age. 
Their papery shells of carbonate of lime are found in great num- 
bers from this time on. 




FIG. 273. Gastropods, Ordovician Species 

Cephalopods, the most highly organized of the mollusks, started 
into existence, so far as the record shows, toward the end of 
the Cambrian, with the 
long extinct OrtJioceras 
(straigJithorri) and the 
allied genera of its 
family. The Orthoceras 
had a long, straight, and 
tapering shell, divided 
by cross partitions into 
chambers. The animal 
lived in the "body 
chamber" at the larger 
end, and walled off the 
other chambers from it 
in succession during the growth of the shell A central tube. 




FIG. 274. Cambrian Pteropods 



326 



THE ELEMENTS OF GEOLOGY 



the siplmncle (s, Fig. 275, B\ passed through from the body 

chamber to the closed tip of the cone. 

The seashells, both brachiopods 
and mollusks, are in some respects 
the most important to the geologist 
of all fossils. They have been so 
numerous, so widely distributed, 
and so well preserved because of 
their durable shells and their 
station in growing sediments, that 
better than any other group of 
organisms they can be used to cor- 
relate the strata of different regions 
and to mark by their slow changes 
the advance of geological time. 

Climate. The life of Cambrian 
times in different countries con- 
tains no suggestion of any marked 

climatic zones, and as in later periods a warm climate probably 

reached to the polar regions. 




B 

FIG. 275. Orthoceras 
A, fossil shell; B, restoration 



CHAPTER XVII 

THE ORDOVICIAN * AND SILURIAN 

THE ORDOVICIAN 

In North America the Ordovician rocks lie conformably on 
the Cambrian. The two periods, therefore, were not parted by 
any deformation, either of mountain making or of continental 
uplift. The general submergence which marked the Cambrian 
continued into the succeeding period with little interruption. 

Subdivisions and distribution of strata. The Ordovician 
series, as they have been made out in New York, are given for 
reference in the following table, with the rocks of which they 
are chiefly composed : 

5 Hudson shales 

4 Utica shales 

3 Trenton limestones 

2 Chazy limestones 

1 Calciferous sandy limestones 

These marine formations of the Ordovician outcrop about the 
Cambrian and pre-Cambrian areas, and, as borings show, extend 
far and wide over the interior of the continent beneath more 
recent strata. The Ordovician sea stretched from Appalachia 
across the Mississippi valley. It seems to have extended to 
California, although broken probably by several mountainous 
islands in the west. 

Physical geography. The physical history of the period is 
recorded in the succession of its formations. The sandstones of 
the Upper Cambrian, as we have learned, tell of a transgressing 

i Often known as the Lower Silurian. 
327 



328 



THE ELEMENTS OF GEOLOGY 



sea which gradually came to occupy the Mississippi valley and 
the interior of North America. The limestones of the early and 
middle Ordovician show that now the shore had become remote 
and the lands had become more low. The waters now had 
cleared. Colonies of brachiopods and other lime-secreting ani- 
mals occupied the sea bottom, and their debris mantled it with 
sheets of limy ooze. The sandy limestones of the Calciferous 

record the transition 
stage from the Cam- 
brian when some sand 
was still brought in 
from shore. The highly 
fossilif erous limestones 
of the Trenton tell of 
clear water and abun- 
dant life. We need not 
regard this epiconti- 
nental sea as deep. No 
abysmal deposits have 
been found, and the 
limestones of the period 
are those which would 
be laid in clear, warm 
water of moderate 
depth, like that of 
modern coral seas. 
The shales of the Utica and Hudson show that the waters of 
the sea now became clouded with mud washed in from land. 
Either the land was gradually uplifted, or perhaps there had 
arrived one of those periodic crises which, as we may imagine, 
have taken place whenever the crust of the shrinking earth has 
slowly given way over its great depressions, and the ocean has 
withdrawn its waters into deepening abysses. The land was 
thus left relatively higher and bordered with new coastal plains. 




FIG. 276. Hypothetical Map of the Eastern 
United States in Ordovician Time 

Shaded areas, probable sea; broken lines, ap- 
proximate shore lines 



THE ORDOVICIAN AND SILURIAN 329 

The epicontinental sea was shoaled and narrowed, and muds 
were washed in from the adjacent lands. 

The Taconic deformation. The Ordovician was closed by a 
deformation whose extent and severity are not yet known. 
From the St. Lawrence Eiver to New York Bay, along the 
northwestern and western border of New England, lies a belt of 
Cambrian-Ordovician rocks more than a mile in total thickness, 
which accumulated during the long ages of those periods in a 
gradually subsiding trough between the Adirondacks and a pre- 
Carnbrian range lying west of the Connecticut River. But since 
their deposition these ancient sediments have been crumpled 
and crushed, broken with great faults, and extensively metamor- 
phosed. The limestones have recrystallized into marbles, among 
them the famous marbles of Vermont ; the Cambrian sandstones 
have become quartzites, and the Hudson shale has been changed 
to a schist exposed on Manhattan Island and northward. 

In part these changes occurred at the close of the Ordovician, 
for in several places beds of Silurian age rest unconformably on 
the upturned Ordovician strata ; but recent investigations have 
made it probable that the crustal movements recurred at later 
times, and it was perhaps in the Devonian and at the close of 
the Carboniferous that the greater part of the deformation and 
metamorphism was accomplished. As a result of these move- 
ments, perhaps several times repeated, a great mountain 
range was upridged, which has been long since leveled by ero- 
sion, but whose roots are now visible in the Taconic Mountains 
of western New England. 

The Cincinnati anticline. Over an oval area in Ohio, Indiana, and 
Kentucky, whose longer axis extends from north to south through 
Cincinnati, the Ordovician strata rise in a very low, broad swell, called 
the Cincinnati anticline. The Silurian and Devonian strata thin out as 
they approach this area and seem never to have deposited upon it. We 
may regard it, therefore, as an island upwarped from the sea at the 
close of the Ordovician or shortly after. 



330 



THE ELEMENTS OF GEOLOGY 



Petroleum and natural gas. These valuable illuminants and 
fuels are considered here because, although they are found in 
traces in older strata, it is in the Ordovician that they occur 
for the first time in large quantities. They range throughout 
later formations down to the most recent. 

The oil horizons of California and Texas are Tertiary ; those of Col- 
orado, Cretaceous; those of West Virginia, Carboniferous; those of 
Pennsylvania, Kentucky, and Canada, Devonian ; and the large field 
of Ohio and Indiana belongs to the Ordovician and higher systems. 

Petroleum and natural gas, wherever found, have probably 
originated from the decay of organic matter when buried in 
sedimentary deposits, just as at present in swampy places the 
hydrogen and carbon of decaying vegetation combine to form 
marsh gas. The light and heat of these hydrocarbons we may 
think of, therefore, as a gift to the civilized life of our race from 
d d' d" the humble organisms, both 

animal and vegetable, of the 
remote past, whose remains 
were entombed in the sedi- 
ments of the Ordovician and 
later geological ages. 
FIG. 277. Diagram illustrating the Con- Petroleum is very widely 
ditions of Accumulation of Oil and disseminated throughout 

the stratified rocks. Certain 

swun^, b, reservoir; c, cover. What ,. --11 

would be the result of boring to the limestones are visibly greasy 

reservoir rock at d? at d'? at d"? w i tri it, and others give off 

its characteristic fetid odor when struck with a hammer. Many 
shales are bituminous, and some are so highly charged that 
small flakes may be lighted like tapers, and several gallons of 
oil to the ton may be obtained by distillation. 

But oil and gas are found in paying quantities only when cer- 
tain conditions meet : 

1. A source below, usually a bituminous shale, from whose 
organic matter they have been derived by slow change. 




Gas 
a, source 



THE ORDOVICIAN AND SILURIAN 331 

2. A reservoir above, in which they have gathered. This is 
either a porous sandstone or a porous or creviced limestone. 

3. Oil and gas are lighter than water, and are usually under 
pressure owing to artesian water. Hence, in order to hold them 
from escaping to the surface, the reservoir must have the shape 
of an anticline, dome, or lens. 

4. It must also have an impervious cover, usually a shale. 
In these reservoirs gas is under a pressure which is often 

enormous, reaching in extreme cases as high as a thousand five 
hundred pounds to the square inch. When tapped it rushes 
out with a deafening roar, sometimes flinging the heavy drill 
high in air. In accounting for this pressure we must remember 
that the gas has been compressed within the pores of the reser- 
voir rock by artesian water, and in some cases also by its own 
expansive force. It is not uncommon for artesian water to rise 
in wells after the exhaustion of gas and oil 

Life of the Ordovician 

During the ages of the Ordovician, life made great advances. 
Types already present branched widely into new genera and 
species, and new and higher 
types appeared. 

Sponges continued from 
the Cambrian. Graptolites 
now reached their climax. 

Stromatopora colonies 
of minute hydrozoans allied 
to corals grew in places 

on the sea floor, secreting 

FIG. 278. Stromatopora 
stony masses composed of 

thin, close, concentric layers, connected by vertical rods. The 
Stromatopora are among the chief limestone builders of the 
Silurian and Devonian periods. 




332 



THE ELEMENTS OF GEOLOGY 



Corals developed along several distinct lines, 
like modern corals they secreted a calcareous 
framework, in whose outer portions the polyps 
lived. In the Ordovician, corals were represented 
chiefly by the family of the Chcetetes, all species 
of which are long since extinct. The description 
of other types of corals will be given under the 
Silurian, where they first became abundant. 

Echinoderms. The cystoid reaches its climax, but 
there appear now two higher types of echinoderms, 
the crinoid and the starfish. The crinoid, named 
from its resemblance to the lily, is like the cystoid 
in many respects, but has a longer stem and sup- 
ports a crown of plumose arms. Stirring the water 
with these arms, it creates currents by which par- 
ticles of food are wafted to its mouth. Crinoids are 
rare at the present time, but they grew in the 
greatest profusion in the warm Ordovician seas and 
for long ages thereafter. In many places the sea 
floor was beautiful with these graceful, flowerlike 
forms, as with fields of long-stemmed lilies. Of the 
higher, free-moving classes of the echinoderms, star- 
fish are more numerous than in the Cambrian, and 
sea urchins make their appearance in rare ar- 
chaic forms. 

Crustaceans. Trilobites now reach their great- 
est development and more than eleven hundred 
species have been described from the rocks of 
this period. It is interesting to note that in 
many species the segments of the thorax have 
now come to be so shaped that they move freely 
on one another. Unlike their Cambrian ances- 
tors, many of the Ordovician trilobites could 
FIG 279 Crinc t ro ^ themselves into balls at the approach of 
Jurassic Species danger. It is in this attitude, taken at the 




THE ORDOVICIAN AND SILURIAN 



333 



approach of death, that trilobites are often found in the Ordo- 
vician and later rocks. The gigantic crustaceans called the 
eurypterids were also present in this 
period (Fig. 282). 





FIG. 280. An Ordo- 
vician Starfish 



FIG. 281. An Ordovi- 
cian Sea Urchin 




FIG. 282. Eurypterus 



The arthropods had now seized upon the land. Centipedes 
and insects of a low type, the earliest known land animals, have 
been discovered in strata of this system. 

Bryozoans. No fossils are more common 
in the limestones of the time than the small 
branching stems and lacelike mats of the 
bryozoans, the skeletons of colonies of a 
minute animal allied in structure to the 
brachiopod. 

Brachiopods. These multiplied greatly, 
and in places their shells formed thick beds of coquina. They 
still greatly surpassed the mollusks in numbers. 




FIG. 283. A Bryozoan 




FIG. 284. Ordovician Brachiopods 

Cephalopods. Among the mollusks we must note the evolu- 
tion of the cephalopods. The primitive straight Orthoceras has 



334 



THE ELEMENTS OF GEOLOGY 




ABC 
FIG. 285. A, Cyrtoceras; B, Trochoceras; 
(7, Lituites 



now become abundant. But in addition to this ancestral type 
there appears a succession of forms more and more curved and 

closely coiled, as 
illustrated in Figure 
285. The nautilus, 
which began its 
course in this 
period, crawls on 
the bottom of our 
present seas. 

Vertebrates. The 
most important 
record of the Ordo- 
vician is that of the 
appearance of a new and higher type, with possibilities of 
development lying hidden in its structure that the mollusk 
and the insect could never hope to reach. Scales and plates 
of minute fishes found in the Ordovician rocks near Canon 
City, Colorado, show that the hum- 
blest of the vertebrates had already 
made its appearance. But it is prob- 
able that vertebrates had been on 
the earth for ages before this in 
lowly types, which, being destitute 
of hard parts, would leave no record. 

THE SILURIAN 

The narrowing of the seas and the 
emergence of the lands which char- 
acterized the closing epoch of the 

FIG. 286. Nautilus 
Ordovician in eastern North America 

continue into the succeeding period of the Silurian. New spe- 
cies appear and many old species now become extinct. 




THE ORDOVICIAN AND SILURIAN 335 

The Appalachian region. Where the Silurian system is most 
fully developed, from New York southward along the Appala- 
chian Mountains, it comprises four series : 

4 Salina . . N . shales, impure limestones, gypsum, salt 

3 Niagara . . . chiefly limestones 

2 Clinton . . . sandstones, shales, with some limestones 

1 Medina . . . conglomerates, sandstones 

The rocks of these series are shallow-water deposits and 
reach the total thickness of some five thousand feet. Evidently 
they were laid over an area which was on the whole gradually 
subsiding, although with various gentle oscillations which are 
recorded in the different formations. The coarse sands of the 
heavy Medina formations record a period of uplift of the old- 
land of Appalachia, when erosion went on rapidly and coarse 
waste in abundance was brought down from the hills by swift 
streams and spread by the waves in wide, sandy flats. As the 
lands were worn lower the waste became finer, and during 
an epoch of transition the Clinton there were deposited 
various formations of sandstones, shales, and limestones. The 
Niagara limestones testify to a long epoch of repose, when low- 
lying lands sent little waste down to the sea. 

The gypsum and salt deposits of the Salina show that toward 
the close of the Silurian period a slight oscillation brought the 
sea floor nearer to the surface, and at the north cut off exten- 
sive tracts from the interior sea. In these wide lagoons, which 
now and then regained access to the open sea and obtained new 
supplies of salt water, beds of salt and gypsum were deposited 
as the briny waters became concentrated by evaporation under 
a desert climate. Along with these beds there were also laid 
shales and impure limestones. 

In New York the " salt pans " of the Salina extended over an area 
one hundred and fifty miles long from east to west and sixty miles wide, 
and similar salt marshes occurred as far west as Cleveland, Ohio, and 



336 THE ELEMENTS OF GEOLOGY 

Goderich on Lake Huron. At Ithaca, New York, the series is fifteen hun- 
dred feet thick, and is buried beneath an equal thickness of later strata. 
It includes two hundred and fifty feet of solid salt, in several distinct 
beds, each sealed within the shales of the series. 

Would you expect to find ancient beds of rock salt inclosed in beds 
of pervious sandstone ? 

The salt beds of the Salina are of great value. They are reached by 
well borings, and their brines are evaporated by solar heat and by boil- 
ing. The rock salt is also mined from deep shafts. 

Similar deposits of salt, formed under like conditions, occur in the 
rocks of later systems down to the present. The salt beds of Texas are 
Permian, those of Kansas are Permian, and those of Louisiana are 
Tertiary. 

The Mississippi valley. The heavy near-shore formations of 
the Silurian in the Appalachian region thin out toward the west. 
The Medina and the Clinton sandstones are not found west of 
Ohio, where the first passes into a shale and the second into a 
limestone. The Niagara limestone, however, spreads from the 
Hudson Eiver to beyond the Mississippi, a distance of more than 
a thousand miles. During the Silurian period the Mississippi 
valley region was covered with a quiet, shallow, limestone- 
making sea, which received little waste from the low lands 
which bordered it. 

The probable distribution of land and sea in eastern North 
America and western Europe is shown in Figure 287. The 
fauna of the interior region and of eastern Canada are closely 
allied with that of western Europe, and several species are 
identical. We can hardly account for this except by a shallow- 
water connection between the two ancient epicontinental seas. 
It was perhaps along the coastal shelves of a northern land con- 
necting America and Europe by way of Greenland and Iceland 
that the migration took place, so that the same species came to 
live in Iowa and in Sweden. 

The western United States. So little is found of the rocks of 
the system west of the Missouri Eiver that it is quite probable 



THE ORDOVICIAN AND SILURIAN 



337 



that the western part of the United States had for the most 
part emerged from the sea at the close of the Ordovician and 




FIG. 287. Hypothetical Map of Parts of North America and 
Europe in Silurian Time 

Shaded areas, probable seas ; broken lines, approximate 
shore lines 

remained land during the Silurian. At the same time the 
western land was perhaps connected with the eastern land of 





FIG. 288. A Compound Cup Coral 



FIG. 289. A Simple 
Cup Coral 



Appalachia across Arkansas and Mississippi; for toward the 
south the Silurian sediments indicate an approach to shore. 



338 



THE ELEMENTS OF GEOLOGY 

Life of the Silurian 



In this brief sketch it is quite impossible to relate the many 
changes of species and genera during the Silurian. 

Corals. Some of the more common types are familiarly 
known as cup corals, honeycomb corals, and chain corals. In 





FIG. 290. Honeycomb Corals 

the cup corals the most important feature is the development 
of radiating vertical partitions, or septa, in the cell of the polyp. 
Some of the cup corals grew in hemispherical colonies (Fig. 288), 
while many were separate individuals (Fig. 289), building a 





FIG. 291. A Chain Coral FIG. 292. A Syringopora Coral 

single conical, or horn-shaped cell, which sometimes reached 
the extreme size of a foot in length and two or three inches in 
diameter. 



THE ORDOVICIAN AND SILURIAN 



339 



Honeycomb corals consist of masses of small, close-set pris- 
matic cells, each crossed by horizontal partitions, or tabulae, 
while the septa are rudimentary, being represented by faintly 
projecting ridges or rows of spines. 

Chain corals are also marked by tabulae. Their cells form 
elliptical tubes, touching each other at the edges, and appearing 
in cross section like the links 
of a chain. They became ex- 
tinct at the end of the Silurian. 

The corals of the Syrinyo- 
pora family are similar in 




B 



FIG. 293. A Blastoid : A, side view, 
showing portion of the stem ; 
B, summit of calyx (species Car- 
boniferous) 




FIG. 294. A Silurian Scorpion 



structure to chain corals, but the tubular columns are con- 
nected only in places. 

To the echinoderms there is now added the Uastoid (bud- 
shaped). The blastoid is stemmed and armless, and its globular 
" head " or " calyx," with its five petal-like divisions, resembles 
a flower bud. The blastoids became more abundant in the 
Devonian, culminated in the Carboniferous, and disappeared at 
the end of the Paleozoic. 

The great eurypterids some of which were five or six feet 
in length and the cephalopods were still masters of the seas. 



340 THE ELEMENTS OF GEOLOGY 

Fishes were as yet few and small ; trilobites and graptolites 
had now passed their prime and had diminished greatly in num- 
bers. Scorpions are found in this period both in Europe and in 
America. The limestone-making seas of the Silurian swarmed 
with corals, crinoids, and brachiopods. 




FIG. 295. Block of Limestone showing Interior Casts of Penlamerus 
oblongus, a Common Silurian Brachiopod 

With the end of the Silurian period the Age of Invertebrates 
comes to a close, giving place to the Devonian, the Age of 
Fishes. 






CHAPTEE XVIII 
THE DEVONIAN 

In America the Silurian is not separated from the Devonian 
by any mountain-making deformation or continental uplift. The 
one period passed quietly into the other. Their conformable 
systems are so closely related, and the change in their faunas 
is so gradual, that geologists are not agreed as to the precise 
horizon which divides them. 

Subdivisions and physical geography. The Devonian is rep- 
resented in New York and southward by the following five 
series. We add the rocks of which they are chiefly composed. 

5 Chemung sandstones and sandy shales 

4 Hamilton shales and sandstones 

3 Corniferous limestones 

2 Oriskany sandstones 

1 Helderberg limestones 

The Helderberg is a transition epoch referred by some geol- 
ogists to the Silurian. The thin sandstones of the Oriskany 
mark an epoch when waves worked over the deposits of for- 
mer coastal plains. The limestones of the Corniferous testify 
to a warm and clear wide sea which extended from the Hud- 
son to beyond the Mississippi. Corals throve luxuriantly, and 
their remains, with those of mollusks and other lime-secret- 
ing animals, built up great beds of limestone. The bordering 
continents, as during the later Silurian, must now have been 
monotonous lowlands which sent down little of even the finest 
waste to the sea. 

In the Hamilton the clear seas of the previous epoch became 
clouded with mud. The immense deposits of coarse sandstones 

341 



342 THE ELEMENTS OF GEOLOGY 

and sandy shales of the Chemung, which are found off what was 
at the time the west coast of Appalachia, prove an uplift of 
that ancient continent. 

The Chemung series extends from the Catskill Mountains to north- 
eastern Ohio and south to northeastern Tennessee, covering an area of 
not less than a hundred thousand square miles. In eastern New York it 
attains three thousand feet in thickness ; in Pennsylvania it reaches 
the enormous thickness of two miles ; but it rapidly thins to the west. 
Everywhere the Chemung is made of thin beds of rapidly alternating 
coarse and fine sands and clays, with an occasional pebble layer, and 
hence is a shallow-water deposit. The fine material has not been thor- 
oughly winnowed from the coarse by the long action of strong waves 
and tides. The sands and clays have undergone little more sorting than 
is done by rivers. We must regard the Chemung sandstones as deposits 
made at the mouths of swift, turbid rivers in such great amount that 
they could be little sorted and distributed by waves. 

Over considerable areas the Chemung sandstones bear little or no 
trace of the action of the sea. The Catskill Mountains, for example, 
have as their summit layers some three thousand feet of coarse red 
sandstones of this series, whose structure is that of river deposits, and 
whose few fossils are chiefly of fresh-water types. The Chemung is 
therefore composed of delta deposits, more or less worked over by the 
sea. The bulk of the Chemung equals that of the Sierra Nevada Moun- 
tains. To furnish this immense volume of sediment a great mountain 
range, or highland, must have been upheaved where the Appalachian 
lowland long had been. To what height the Devonian mountains of 
Appalachia attained cannot be told from the volume of the sediments 
wasted from them, for they may have risen but little faster than they 
were worn down by denudation. We may infer from the character of 
the waste which they furnished to the Chemung shores that they did 
not reach an Alpine height. The grains of the Chemung sandstones 
are not those which would result from mechanical disintegration, as by 
frost on high mountain peaks, but are rather those which would be left 
from the long chemical decay of siliceous crystalline rocks ; for the more 
soluble minerals are largely wanting. The red color of much of the 
deposits points to the same conclusion. Red residual clays accumulated 
on the mountain sides and upland summits, and were washed as ocherous 
silt to mingle with the delta sands. The iron-bearing igneous rocks 



THE DEVONIAN 343 

of the oldland also contributed by their decay iron in solution to the 
rivers, to be deposited in films of iron oxide about the quartz grains of 
the Chemung sandstones, giving them their reddish tints. 

LIFE OF THE DEVONIAN 

Plants. The lands were probably clad with verdure during 
Silurian times, if not still earlier ; for some rare remains of ferns 
and other lowly types of vegetation have been found in the 
strata of that system. But it is in the Devonian that we dis- 
cover for the first time the remains of extensive and luxuriant 
forests. This rich flora reached its climax in the Carboniferous, 
and it will be more convenient to describe its varied types in 
the next chapter. 

Rhizocarps. In the shales of the Devonian are found micro- 
scopic spores of rhizocarps in such countless numbers that their 
weight must be reckoned in hundreds of millions of tons. It 
would seem that these aquatic plants culminated in this period, 
and in widely distant portions of the earth swampy flats and 
shallow lagoons were filled with vegetation of this humble type, 
either growing from the bottom or floating free upon the sur- 
face. It is to the resinous spores of the rhizocarps that the 
petroleum and natural gas from Devonian rocks are largely 
due. The decomposition of the spores has made the shales 
highly bituminous, and the oil and gas have accumulated in 
the reservoirs of overlying porous sandstones. 

Invertebrates. We must pass over the ever-changing groups 
of the invertebrates with the briefest notice. Chain corals 
became extinct at the close of the Silurian, but other corals 
were extremely common in the Devonian seas. At many 
places corals formed thin reefs, as at Louisville, Kentucky, 
where the hardness of the reef rock is one of the causes of the 
Falls of the Ohio. 

Sponges, echinoderms, brachiopods, and mollusks were abun- 
dant. The cephalopods take a new departure. So far in all 




344 THE ELEMENTS OF GEOLOGY 

their various forms, whether straight, as the Orthoceras, or 
curved, or close-coiled as in the nautilus, the septum, or parti- 
tion dividing the chambers, met the inner shell along a simple 
line, like that of the rim of a saucer. There now begins a 

growth of the septum by which its 
edges become sharply corrugated, and 
the suture, or line of juncture of the 
septum and the shell, is thus angled. 
The group in which this growth of 
the septum takes place is called the 
Goniatite (Greek gonia, angle). 

Vertebrates. It is with the great- 
est interest that we turn now to 
study the backboned animals of the 
Devonian; for they are believed to 

be the ancestors of the hosts of verte- 
FIG. 296. A Goniatite 

brates which have since dominated 

the earth. Their rudimentary structures foreshadowed what 
their descendants were to be, and give some clue to the earliest 
vertebrates from which they sprang. Like those whose remains 
are found in the lower Paleozoic systems, all of 
these Devonian vertebrates were aquatic and go 
under the general designation of fishes. 

The lowest in grade and nearest, perhaps, to the 
ancestral type of vertebrates, was the problematic 
creature, an inch or so long, of Figure 297. Note 
the circular mouth not supplied with jaws, the lack 
of paired fins, and the symmetric tail fin, with 
the column of cartilaginous, ringlike vertebrse run- FIG. 297. 
ning through it to the end. The animal is prob- 
ably to be placed with the jawless lampreys and 
hags, a group too low to be included among true fishes. 

Ostracoderms. This archaic group, long since extinct, is 
also too lowly to rank among the true fishes, for its members 



THE DEVONIAN 345 

have neither jaws nor paired fins. These small, fishlike forms 
were eased in front with bony plates developed in the skin and 
covered in the rear with scales. The vertebrae were not ossified, 
for no trace of them has been found. 




FIG. 298. An Ostracoderm 

Devonian fishes. The true fishes of the Devonian can best 
be understood by reference to their descendants now living. 
Modern fishes are divided into several groups: sharks and 
their allies ; dipnoans; ganoids, such as the sturgeon and gar ; 
and teleosts, most common fishes, such as the perch and cod. 

Sharks. Of all groups of living fishes the sharks are the 
oldest and still retain most fully the embryonic characters of 
their Paleozoic ancestors. Such characters are the cartilaginous 




FIG. 299. A Paleozoic Shark 

skeleton, and the separate gill slits with which the throat wall 
is pierced and which are arranged in line like the gill openings 
of the lamprey. The sharks of the Silurian and Devonian are 
known to us chiefly by their teeth and fin spines, for they were 
unprotected by scales or plates, and were devoid of a bony 



346 THE ELEMENTS OF GEOLOGY 

skeleton. Figure 299 is a restoration of an archaic shark from 
a somewhat higher horizon. Note the seven gill slits and the 
lappetlike paired fins. These fins seem to be remnants of the 
continuous fold of skin which, as embryology teaches, passed 
from fore to aft down each side of the primitive vertebrate. 

Devonian sharks were comparatively small. They had not 
evolved into the ferocious monsters which were later to be 
masters of the seas. 

Dipnoans, or lung fishes. These are represented to-day by a 
few peculiar fishes and are distinguished by some high structures 
which ally them with amphibians. An air sac with cellular 
spaces is connected with the gullet and serves as a rudimen- 
tary lung. It corresponds with the swim bladder of most mod- 




FIG. 300. A Devonian Dipnoan 

ern fishes, and appears to have had a common origin with it. 
We may conceive that the primordial fishes not only had gills 
used in breathing air dissolved in water, but also developed a 
saclike pouch off the gullet. This sac evolved along two dis- 
tinct lines. On the line of the ancestry of most modern fishes 
its duct was closed and it became the swim bladder used in 
flotation and balancing. On another line of descent it was left 
open, air was swallowed into it, and it developed into the rudi- 
mentary lung of the dipnoans and into the more perfect lungs 
of the amphibians and other air-breathing vertebrates. 

One of the ancient dipnoans is illustrated in Figure 300. 
Some of the members of this order were, like the ostraco- 
derms, cased in armor, but their higher rank is shown by their 
powerful jaws and by other structures. Some of these armored 



THE DEVONIAN 347 

fishes reached twenty-five feet in length and six feet across the 
head. They were the tyrants of the Devonian seas. 

Ganoids. These take their name from their enameled plates 
or scales of bone. The few genera now surviving are the 
descendants of the tribes which swarmed in the Devonian seas. 
A restoration of one of a leading order, the fringe-finned 
ganoids, is given in Figure 301. The side fins, which corre- 
spond to the limbs of the higher vertebrates, are quite unlike 
those of most modern fishes. Their rays, instead of radiating 
from a common base, fringe a central lobe which contains a 




FIG. 301. A Devonian Fringe-Finned Ganoid 

cartilaginous axis. The teeth of the Devonian ganoids show 
a complicated folded structure. 

General characteristics of Devonian fishes. The notochord 
is persistent. The notochord is a continuous rod of cartilage, or 
gristle, which in the embryological growth of vertebrate ani- 
mals supports the spinal nerve cord before the formation of 
the vertebrae. In most modern fishes and in all higher verte- 
brates the notochord is gradually removed as the bodies of the 
vertebrae are formed about it ; but in the Devonian fishes it 
persists through maturity and the vertebrae remain incomplete. 

The skeleton is cartilaginous. This also is an embryological 
characteristic. In the Devonian fishes the vertebrae, as well as 
the other parts of the skeleton, have not ossified, or changed to 
bone, but remain in their primitive cartilaginous condition. 



348 



THE ELEMENTS OF GEOLOGY 



The tail fin is vertebrated. The backbone runs through the 
fin and is fringed above and below with its vertical rays. In 
some fishes with vertebrated tail fins the fin is symmetric 
(Fig. 300), and this seems to be the primitive type. In others 
the tail fin is unsymmetric : the backbone runs into the upper 
lobe, leaving the two lobes of unequal size. In most modern 

fishes (the teleosts) 
the tail fin is not 
vertebrated: the 
spinal column ends 
in a broad plate, to 
which the diver- 
ging fin rays are 
attached. 

But along with 
these embryonic 
characters, which 
were common to all 
Devonian fishes, 
there were other 
structures in certain groups which foreshadowed the higher 
structures of the land vertebrates which were yet to come : air 
sacs which were to develop into lungs, and cartilaginous axes in 
the side fins which were a prophecy of limbs. The vertebrates 
had already advanced far enough to prove the superiority of 
their type of structure to all others. Their internal skeleton 
afforded the best attachment for muscles and enabled them to 
become the largest and most powerful creatures of the time. 
The central nervous system, with the predominance given to the 
ganglia at the fore end of the nerve cord, the brain, already 
endowed them with greater energy than the invertebrates ; and, 
still more important, these structures contained the possibility 
of development into the more highly organized land vertebrates 
which were to rule the earth. 




FIG. 302. Vertebrae of Sturgeon in side view A ; 
and vertical transverse section B, showing 
Notochord ch, and Neural Canal m 



THE DEVONIAN 349 

Teleosts. The great group of fishes called the teleosts, or those with 
complete bony skeletons, to which most modern fishes belong, may be 
mentioned here, although in the Devonian they had not yet appeared. 
The teleosts are a highly specialized type, adapted most perfectly to 
their aquatic environment. Heavy armor has been discarded, and 
reliance is placed on swiftness instead. The skeleton is completely 
ossified and the notochord removed. The vertebrae have been eco- 
nomically withdrawn from the tail, and the cartilaginous axis of the 
side fins has been found unnecessary. The air sac has become a swim 
bladder. In this complete specialization they have long since lost the 
possibility of evolving into higher types. 

It is interesting to note that the modern teleosts in their embryo- 
logical growth pass through the stages which characterized the maturity 
of their Devonian ancestors ; their skeleton is cartilaginous and their 
tail fin vertebrated. 

Vertebrates come to land. A footprint, the oldest known, 
found in Upper Devonian rocks of Pennsylvania, shows that 
vertebrates then trod the land, and fishes had developed into 
amphibians. The goal of the land was won, it seems, by the 
fringe-filmed ganoids, who lived not in the sea but in the 
streams and lakes of semi-arid regions. In dry seasons these 
streams dwindled to stagnant pools or withered quite away. 
Hence these fishes were compelled to use their air sacs more 
and more as lungs, gulping down air to supplement that taken 
by their gills from the stagnant, overcrowded water. Mean- 
while they were able to crawl about, when streams went dry, 
by means of their side fins, whose cartilaginous supports were 
developing into strong limb bones. The Devonian fringe-finned 
ganoids are supposed to have been the ancestors of the first 
amphibia, not only because their fins and air sacs could be 
adapted to life on land, but also because they w r ere very like 
early amphibia in the bones of the skull and the infolded 
structure of the teeth. 



CHAPTEE XIX 
THE CARBONIFEROUS 

The Carboniferous system is so named from the large amount 
of coal which it contains. Other systems, from the Devonian 
on, are coal bearing also, but none so richly and to so wide an 
extent. Never before or since have the peculiar conditions been 
so favorable for the formation of extensive coal deposits. 

With few exceptions the Carboniferous strata rest on those 
of the Devonian without any marked unconformity ; the one 
period passed quietly into the other, with no great physical 
disturbances. 

The Carboniferous includes three distinct series. The lower is 
called the Mississippian, from the outcrop of its formations along 
the Mississippi River in central and southern Illinois and the 
adjacent portions of Iowa and Missouri. The middle series is 
called the Pennsylvanian (or Coal Measures), from its wide 
occurrence over Pennsylvania. The upper series is named the 
Permian, from the province of Perm in Russia. 

The Mississippian series. In the interior the Mississippian 
is composed chiefly of limestones, with some shales, which tell 
of a clear, warm, epicontinental sea swarming with crinoids, 
corals, and shells, and occasionally clouded with silt from the 
land. 

In the eastern region, New York had been added by uplift 
to the Appalachian land which now was united to the northern 
area. From eastern Pennsylvania southward there were laid in a 
subsiding trough, first, thick sandstones (the Pocono sandstone), 
and later still heavier shales, the two together reaching the 
thickness of four thousand feet and more. We infer a renewed 

350 



rTHE CARBONIFEROUS 351 

uplift of Appalachia similar to that of the later epochs of the 
Devonian, but as much less in amount as the volume of sedi- 
ments is smaller. 

THE PENNSYLVANIA^ SEEIES 

The Mississippian was brought to an end by a quiet oscilla- 
tion which lifted large areas slightly above the sea, and the 
Pennsylvanian began with a movement in the opposite direction. 
The sea encroached on the new land, and spread far and wide 
a great basal conglomerate and coarse sandstones. On this 
ancient beach deposit a group of strata rests which we must 
now interpret. They consist of alternating shales and sand- 
stones, with here and there a bed of limestone and an occa- 
sional seam of coal. A stratum of fire clay commonly underlies 
a coal seam, and there occur also beds of iron ore. We give 
a typical section of a very small portion of the series at a local- 
ity in Pennyslvania. Although some of the minor changes are 
omitted, the section shows the rapid alternation of the strata : 

Feet 

9 Sandstone and shale 25 

8 Limestone 18 

7 Sandstone 10 

6 Coal 1-6 

5 Shale 0-2 

4 Sandstone 40 

3 Limestone 10 

2 Coal 5-12 

1 Fire clay ...... 3 

This section shows more coal than is usual; on the whole, 
coal seams do not take up more than one foot in fifty of the 
Coal Measures. They vary also in thickness more than is seen 
in the section, some exceptional seams reaching the thickness 
of fifty feet. 



352 THE ELEMENTS OF GEOLOGY 

How coal was made. 1. Coal is of vegetable origin. Exam- 
ined under the microscope even anthracite, or hard coal, is seen 
to contain carbonized vegetal tissues. There are also all grada- 
tions connecting the hardest anthracite through semibitumi- 
nous coal, bituminous or soft coal, lignite (an imperfect coal 
in which sometimes woody fibers may be seen little changed) 
with peat and decaying vegetable tissues. Coal is compressed 
and mineralized vegetal matter. Its varieties depend on the 
perfection to which the peculiar change called bituminization 
has been carried, and also, as shown in the table below, on the 
degree to which the volatile substances and water have escaped, 
and on the per cent of carbon remaining. 

Peat Lignite Bituminous Coal Anthracite 



Dismal Swamp Texas 


Penn. 


Penn. 


Moisture . . . 


78.89 


14.67 


1.3 


2.74 


Volatile matter . 


13.84 


37.32 


20.87 


4.25 


Fixed carbon 


6.49 


41.07 


67.20 


81.51 


Ash . 


.78 


6.69 


8.80 ' 


10.87 



2. The vegetable remains associated with coal are those of 
land plants. 

3. Coal accumulated in the presence of water ; for it is only 
when thus protected from the air that vegetal matter is pre- 
served. 

4. The vegetation of coal accumulated for the most part 
where it grew ; it was not generally drifted and deposited by 
waves and currents. Commonly the fire clay beneath the seam 
is penetrated with roots, and the shale above is packed with 
leaves of ferns and other plants as beautifully pressed as in a 
herbarium. There often is associated with the seam a fossil 
forest, with the stumps, which are still standing where they 
grew, their spreading roots, and the soil beneath, all changed to 
stone (Fig. 303). In the Nova Scotia field, out of seventy-six 
distinct coal seams, twenty are underlain by old forest grounds. 



THE CARBONIFEROUS 



353 



The presence of fire clay beneath a seam points in the same 
direction. Such underclays withstand intense heat and are used 
in making fire brick, because their alkalies have been removed 
by the long-continued growth of vegetation. 

Fuel coal is also too pure to have been accumulated by 
driftage. In that case we should expect to find it mixed with 
mud, while in fact it often contains no more ash than the 
vegetal matter would furnish from which it has been compressed. 







FIG. 303. Fossil Tree Stumps of a Carboniferous Forest, Scotland 

These conditions are fairly met in the great swamps of river 
ins and deltas and of coastal plains, such as the great Dismal 
Swamp, where thousands of generations of forests with their 
ergrowths contribute their stems and leaves to form thick 
beds of peat. A coal seam is a fossil peat bed. 

Geographical conditions during the Pennsylvanian. The Car- 
iferous peat swamps were of vast extent. A map of the 
Coal Measures (Fig. 260) shows that the coal marshes stretched, 
with various interruptions of higher ground and straits of open 



35-4 THE ELEMENTS OF GEOLOGY 

water, from eastern Pennsylvania into Alabama, Texas, and 
Kansas. Some individual coal beds may still be traced over a 
thousand square miles, despite the erosion which they have 
suffered. It taxes the imagination to conceive that the varied 
region included within these limits was for hundreds of thou- 
sands of years a marshy plain covered with tropical jungles 
such as that pictured in Figure 304. 

On the basis that peat loses four fifths of its bulk in chan- 
ging to coal, we may reckon the thickness of these ancient peat 
beds. Coal seams six and ten feet thick, which are not uncom- 
mon, represent peat beds thirty and fifty feet in thickness, 
while mammoth coal seams fifty feet thick have been com- 
pressed from peat beds two hundred and fifty feet deep. 

At the same time, the thousands of feet of marine and fresh- 
water sediments, with their repeated alternations of limestones, 
sandstones, and shales, in which the seams of coal occur, prove 
a slow subsidence, with many changes in its rate, with halts 
when the land was at a stillstand, and with occasional move- 
ments upward. 

When subsidence was most rapid and long continued the 
sea encroached far and wide upon the lowlands and covered the 
coal swamps with sands and muds and limy oozes. When sub- 
sidence slackened or ceased the land gained on the sea. Bays 
were barred, and lagoons as they gradually filled with mud 
became marshes. Eiver deltas pushed forward, burying with 
their silts the sunken peat beds of earlier centuries, and at the 
surface emerged in broad, swampy flats, like those of the 
deltas of the Mississippi and the Ganges, which soon were 
covered with luxuriant forests. At times a gentle uplift brought 
to sea level great coastal plains, which for ages remained mantled 
with the jungle, their undeveloped drainage clogged with its 
debris, and were then again submerged. 

Physical geography of the several regions. The Acadian 
region lay on the eastern side of the northern land, where now 




365 



356 THE ELEMENTS OF GEOLOGY 

are New Brunswick and Nova Scotia, and was an immense 
river delta. Here river deposits rich in coal accumulated to a 
depth of sixteen thousand feet. The area of this coal field is 
estimated at about thirty-six thousand square miles. 

The Appalachian region skirts the Appalachian oldland on 
the west from the southern boundary of New York to northern 
Alabama, extending west into eastern Ohio. The Cincinnati 
anticline was now a peninsula, and the broad gulf which had lain 
between it and Appalachia was transformed at the beginning of 
the Pennsylvanian into wide marshy plains, now sinking beneath 
the sea and now emerging from it. This area subsided during 
the Carboniferous period to a depth of nearly ten thousand feet. 

The Central region lay west of the peninsula of the Cin- 
cinnati anticline, and extended from Indiana west into eastern 
Nebraska, and from central Iowa and Illinois southward about 
the ancient island in Missouri and Arkansas into Oklahoma 
and Texas. On the north the subsidence in this area was com- 
paratively slight, for the Carboniferous strata scarcely exceed 
two thousand feet in thickness. But in Arkansas and Indian 
Territory the downward movement amounted to four and five 
miles, as is proved by shoal water deposits of that immense 
thickness. 

The coal fields of Indiana and Illinois are now separated by 
erosion from those lying west of the Mississippi Eiver. At the 
south the Appalachian land seems still to have stretched away 
to the west across Louisiana and Mississippi into Texas, and 
this westward extension formed the southern boundary of the 
coal marshes of the continent. 

The three regions just mentioned include the chief Carbon- 
iferous coal fields of North America. Including a field in central 
Michigan evidently formed in an inclosed basin (Fig. 260), and 
one in Ehode Island, the total area of American coal fields has 
been reckoned at not less than two hundred thousand square 
miles. We can hardly estimate the value of these great stores 



THE .CARBONIFEROUS 357 

of fossil fuel to an industrial civilization. The forests of the 
coal swamps accumulated in their woody tissues the energy 
which they received from the sun in light and heat, and it is 
this solar energy long stored in coal seams which now forms 
the world's chief source of power in manufacturing. 

The western area. On the Great Plains beyond the Missouri 
Kiver the Carboniferous strata pass under those of more recent 
systems. Where they reappear, as about dissected mountain 
axes or on stripped plateaus, they consist wholly of marine 
deposits and are devoid of coal. The rich coal fields of the West 
are of later date. 

On the whole the Carboniferous seems to have been a time 
of subsidence in the West. Throughout the period a sea covered 
the Great Basin and the plateaus of the Colorado Eiver. At the 
time of the greatest depression the sites of the central chains of. 
the Rockies were probably islands, but early in the period they 
may have" been connected with the broad lands to the south 
and east. Thousands of feet of Carboniferous sediments were 
deposited where the Sierra Nevada Mountains now stand. 

The Permian. As the Carboniferous period drew toward its 
close the sea gradually withdrew from the eastern part of the 
continent. Where the sea lingered in the deepest troughs, and 
where inclosed basins were cut off from it, the strata of the Per- 
mian were deposited. Such are found in New Brunswick, in 
Pennsylvania and West Virginia, in Texas, and in Kansas. In 
southwestern Kansas extensive Permian beds of rock salt and 
gypsum show that here lay great salt lakes in which these 
minerals were precipitated as their brines grew dense and dried 
away. 

In the southern hemisphere the Permian deposits are so extraordi- 
nary that they deserve a brief notice, although we have so far omitted 
mention of the great events which characterized the evolution of other 
continents than our own. The Permian fauna-flora of Australia, India, 
South Africa, and the southern part of South America are so similar 



358 THE ELEMENTS OF GEOLOGY 

that the inference is a reasonable one that these "widely separated 
regions were then connected together, probably as extensions of a great 
antarctic continent. 

Interbedded with the Permian strata of the first three countries 
named are extensive and thick deposits of a peculiar nature which are 
clearly ancient ground moraines. Clays and sand, now hardened to 
firm rock, are inset with unsorted stones of all sizes, which often are 
faceted and scratched. Moreover, these bowlder clays rest on rock 
pavements which are polished and scored with glacial markings. 
Hence toward the close of the Paleozoic the southern lands of the east- 
ern hemisphere were invaded by ice sheets like that which now shrouds 
Greenland. 

These Permian ground moraines are not the first traces of the work 
of glaciers met with in the geological record. Similar deposits prove 
glaciation in Norway succeeding the pre-Cambrian stage of elevation, 
and Cambrian glacial drift has recently been found in China (page 314). 

The Appalachian deformation. We have seen that during 
Paleozoic times a long, narrow trough of the sea lay off the 
western coast of the ancient land of Appalachia, where now are 
the Appalachian Mountains. During the long ages of this era 
the trough gradually subsided, although with many stillstands 
and with occasional slight oscillations upward. Meanwhile 
the land lying to the east was gradually uplifted at varying 
rates and with long pauses. The waste of the rising land 
was constantly transferred to the sinking marginal sea bottom, 
and on the whole the trough was filled with sediments as 
rapidly as it subsided. The sea was thus kept shallow, and at 
times, especially toward the close of the era, much of the area 
was upbuilt or raised to low, marshy coastal plains. When the 
Carboniferous was ended the waste which had been removed 
from the land and laid along its margin in the successive forma- 
tions of the Paleozoic had reached a thickness of between thirty 
and forty thousand feet. 

Both by sedimentation and by subsidence the trough had 
now become a belt of weakness in the crust of the earth. Here 



THE CARBONIFEROUS 359 

the crust was now made of layers to the depth of six or seven 
miles. In comparison with the massive crystalline rocks of 
Appalachia on the east, the layered rock of the trough was weak 
to resist lateral pressure, as a ream of sheets of paper is weak 
when compared with a solid board of the same thickness. It was 
weaker also than the region to the west, since there the sedi- 
ments were much thinner. Besides, by the long-continued 
depression the strata of the trough had been bent from the 
flat-lying attitude in which they were laid to one in which they 
were less able to resist a horizontal thrust. 

There now occurred one of the critical stages in the history 
of the planet, when the crust crumples under its own weight 
and shrinks down upon a nucleus which is diminishing in vol- 
ume and no longer able to support it. Under slow but resist- 
less pressure the strata of the Appalachian trough were thrust 
against the rigid land, and slowly, steadily bent into long folds 
whose axes ran northeast-southwest parallel to the ancient 
coast line. It was on the eastern side next the buttress of the 
land that the deformation was the greatest, and the folds most 
steep and close. In central Pennsylvania and West Virginia the 
folds were for the most part open. South of these states the folds 
were more closely appressed, the strata were much broken, and 
the great thrust faults were formed which have been described 
already (p. 218). In eastern Pennsylvania seams of bituminous 
coal were altered to anthracite, while outside the region of 
strong deformation, as in western Pennyslvania, they remained 
unchanged. An important factor in the deformation was the 
massive limestones of the Cambrian-Ordovician. Because of 
these thick, resistant beds the rocks were bent into wide folds 
and sheared in places with great thrust faults. Had the strata 
been weak shales, an equal pressure would have crushed and 
mashed them. 

Although the great earth folds were slowly raised, and no 
doubt eroded in their rising, they formed in all probability a 



360 THE ELEMENTS OF GEOLOGY 

range of lofty mountains, with a width of from fifty to a hun- 
dred and twenty-five miles, which stretched from New York 
to central Alabama. 

From their bases lowlands extended westward to beyond the 
Missouri Eiver. At the same time ranges were upridged out 
of thick Paleozoic sediments both in the Bay of Fundy region 
and in the Indian Territory. The eastern portion of the North 
American continent was now well-nigh complete. 

The date of the Appalachian deformation is told in the usual 
way. The Carboniferous strata, nearly two miles thick, are all 
infolded in the Appalachian ridges, while the next deposits 
found in this region those of the later portion of the first 
period (the Trias) of the succeeding era rest unconf ormably on 
the worn edges of the Appalachian folded strata. The deforma- 
tion therefore took place about the close of the Paleozoic. It 
seems to have begun in the Permian, in eastern Pennsylvania, 
for here the Permian strata are wanting, and to have con- 
tinued into the Trias, whose earlier formations are absent over 
all the area. 

With this wide uplift the subsidence of the sea floor which 
had so long been general in eastern North America carne to an 
end. Deposition now gave place to erosion. The sedimentary 
record of the Paleozoic was closed, and after an unknown lapse 
of time, here unrecorded, the annals of the succeeding era were 
written under changed conditions. 

In western North America the closing stages of the Paleozoic 
were marked by important oscillations. The Great Basin, which 
had long been a mediterranean sea, was converted into land 
over western Utah and eastern Nevada, while the waves of the 
Pacific rolled across California and western Nevada. 

The absence of tuffs and lavas among the Carboniferous strata 
of North America shows that here volcanic action was singu- 
larly wanting during the entire period. Even the Appalachian 
deformation was not accompanied by any volcanic outbursts. 



THE CARBONIFEROUS 



361 



LIFE OF THE CARBONIFEROUS 

Plants. The gloomy forests and dense undergrowths of the 
Carboniferous jungles would appear unfamiliar to us could we 
see them as they grew, and even a botanist would find many 
of their forms perplexing and hard to classify. None of our 
modern trees would meet the eye. Plants with conspicuous 





FIG. 305. Carboniferous Ferns 



FIG. 306. Calamites 



flowers of fragrance and beauty were yet to come. Even mosses 
and grasses were still absent. 

Tree ferns lifted their crowns of feathery fronds high in air 
on trunks of woody tissue ; and lowly herbaceous ferns, some 
belonging to existing families, carpeted the ground. Many of 
the fernlike forms, however, have distinct affinities with the 
cycads, of which they may be the ancestors, and some bear seeds 
and must be classed as gymnosperms. 

Dense thickets, like cane or bamboo brakes, were composed 
of thick clumps of Calamites, whose slender, jointed stems shot 
up to a height of forty feet, and at the joints bore slender 



362 



THE ELEMENTS OF GEOLOGY 



branches set with whorls of leaves. These were close allies of the 
Equiseta or " horsetails," of the present ; but they bore character- 
istics of higher classes in the woody structures of their stems. 

There were also vast monotonous forests, composed chiefly of 
trees belonging to the lycopods, and whose nearest relatives 
to-day are the little club mosses of our eastern woods. Two 





FIG. 307. Lepidodendron 



FIG. 308. Sigillaria 



families of lycopods deserve special mention, the Lepidoden- 
drons and the Sigillaria. 

The Lepidodendron, or " scale tree," was a gigantic club moss 
fifty and seventy-five feet high, spreading toward the top into 
stout branches, at whose ends were borne cone-shaped spore 
cases. The younger parts of the tree were clothed with stiff 
needle-shaped leaves, but elsewhere the trunk and branches 
were marked with scalelike scars, left by the fallen leaves, and 
arranged in spiral rows. 

The Sigillaria, or " seal tree," was similar to the Lepidoden- 
dron, but its fluted trunk divided into even fewer branches, and 
was dotted with vertical rows of leaf scars, like the impressions 
of a seal. 



THE CARBONIFEROUS 363 

Both Lepidodendron and Sigillaria were anchored by means 
of great cablelike underground stems, which ran to long dis- 
tances through the marshy ground. The trunks of both trees 
had a thick woody rind, inclosing loose cellular tissue and a 
pith. Their hollow stumps, filled with sand and mud, are com- 
mon in the Coal Measures, and in them one sometimes finds 
leaves and stems, land shells, and the bones of little reptiles of 
the time which made their home there. 

It is important to note that some of these gigantic lycopods, 
which are classed with the cryptogams, or flowerless plants, had 
pith and medullary rays dividing their cylinders into woody 
wedges. These characters connect them with the phanerogams, 
or flowering plants. Like so many of the organisms of the 
remote past, they were connecting types from which groups 
now widely separated have diverged. 

Gymnosperms, akin to the cycads, were also present in the 
Carboniferous forests. Such were the different species of Cor- 
daites, trees pyramidal in shape, with strap-shaped leaves and 
nutlike fruit. Other gymnosperms were related to the yews, 
and it was by these that many of the fossil nuts found in the 
Coal Measures were borne. It is thought by some that the 
gymnosperms had their station on the drier plains and higher 
lands. 

The Carboniferous jungles extended over parts of Europe 
and of Asia, as well as eastern North America, and reached 
from the equator to within nine degrees of the north pole. 
Even hi these widely separated regions the genera and species 
of coal plants are close akin and often identical. 

Invertebrates. Among the echinoderms, crinoids are now 
exceedingly abundant, sea urchins are more plentiful, and sea 
cucumbers are found now for the first time. Trilobites are 
rapidly declining, and pass away forever with the close of the 
period. Eurypterids are common ; stinging scorpions are abun- 
dant ; and here occur the first-known spiders. 



364 



THE ELEMENTS OF GEOLOGY 



We have seen that the arthropods were the first of all 
animals to conquer the realm of the air, the earliest insects 
appearing in the Ordovician. Insects had now become exceed- 
ingly abundant, and the Carboniferous forests swarmed with 
the ancestral types of dragon flies, some with a spread of wing 
of more than tw^o feet, May flies, crickets, and locusts. Cock- 
roaches infested the swamps, and one hundred and thirty-three 

species of this ancient order have 
been discovered in the Carbon- 
iferous of North America. The 
higher flower-loving insects are 
still absent; the reign of the 





FIG. 309. Carboniferous Brachiopods 

A, Productus; B, Spirifer, the right-hand figure showing the interior with the 
calcareous spires for the support of the arms 

flowering plants has not yet begun. The Paleozoic insects were 
generalized types connecting the present orders. Their fore 
wings were still membranous and delicately veined, and used 
in flying; they had not yet become thick, and useful only as 
wing covers, as in many of their descendants. 

Fishes still held to the Devonian types, with the exception 
that the strange ostracoderms now had perished. 

Amphibians. Footprints of amphibians are found in the 
Devonian. The earliest Carboniferous amphibians were small, 
newtlike creatures possessing not only the typical amphibian 
double breathing system of gills and lungs but also a double loco- 
motive apparatus of short, weak legs for crawling on land and a 



THE CARBONIFEROUS 



365 



tail for propulsion in the water. They branched into a variety 
of types, some large and crocodilian, some with well-developed 




FIG. 310. A Carboniferous Dragon Fly 
One tenth natural size 

legs for running, some with large heads like giant tadpoles, 
and some eel-like and limbless. 

The earliest amphibians differ from those of to-day in a 
number of respects. They were connecting 
types Unking together fishes, from which 
they were descended, with reptiles, of which 
they were the ancestors. They retained the 
evidence of their close relationship with the 
Devonian fishes in their cold blood, their 
gills and aquatic habit during their larval 
stage, their teeth with dentine infolded like 
those of the Devonian ganoids but still 
more intricately, and their biconcave ver- 
tebrae which never completely ossified. 
These, the highest vertebrates of the time, 
had not yet advanced beyond the embry- 
onic stage of the more or less cartilagi- 
nous skeleton and the persistent notochord. 




FIG. 311. A Carbon- 
iferous Amphibian 



366 



THE ELEMENTS OF GEOLOGY 



On the other hand, the skull of the Carboniferous amphibians 
was made of close-set bony plates, like the skull of the reptile, 

rather than like that of the frog, 
with its open spaces (Figs. 313 
and 314). Unlike modern amphib- 
ians, with their slimy skin, the 
Carboniferous amphibians wore an 
armor of bony scales over the 
ventral surface and sometimes over 
the back as well. 



-P 




FIG. 312. Transverse Section of 



The footprint which records the ap- 
pearance of the first land vertebrate in 
Devonian times shows a foot with two 
Segment of Tooth of Ancient digits and a rudimentary budding third. 
Amphibian The Carboniferous amphibians had five- 

toed feet, the primitive type of foot from which their descendants of 
higher orders, with a smaller number of digits, have diverged. 

The Carboniferous was the age of lycopods and amphibians, 
as the Devonian had been the age of rhizocarps and fishes. 





FIG. 313. Skull of a Permian Amphibian 
from Texas 



FIG. 314. Skull of a 
Frog 



Life of the Permian. The close of the Paleozoic was, as we 
have seen, a time of marked physical changes. The upridging 
of the Appalachians had begun and a wide continental uplift 
proved by the absence of Permian deposits over large areas 
where sedimentation had gone on before opened new lands 



THE CARBONIFEROUS 367 

for settlement to hordes of air-breathing animals. Changes of 
climate compelled extensive migrations, and the fauna of differ- 
ent regions were thus brought into conflict. The Permian was 
a time of pronounced changes in plant and animal life, and a 
transitional period between two great eras. The somber forests 
of the earlier Carboniferous, with their gigantic club mosses, 
were now replaced by forests of cycads, tree ferns, and conifers. 
Even in the lower Permian the Lepidodendron and Sigillaria 
were very rare, and before the end of the epoch they and the 
Calamites also had become extinct. Gradually the antique 
types of the Paleozoic fauna died out, and in the Permian 
rocks are found the last survivors of the cystoid, the trilobite, 
and the eurypterid, and of many long-lived families of brach- 
iopods, mollusks, and other invertebrates. The venerable Or- 
thoceras and the Goniatite linger on through the epoch and into 
the first period of the succeeding era. Forerunners of the great 
ammonite family of cephalopod mollusks now appear. The 
antique forms of the earlier Carboniferous amphibians continue, 
but with many new genera and a marked increase in size. 

A long forward step had now been taken in the evolution 
of the vertebrates. A new and higher type, the reptiles, had 
appeared, and in such numbers and variety are they found in 
the Permian strata that their advent may well have occurred in 
a still earlier epoch. It will be most convenient to describe the 
Permian reptiles along with their descendants of the Mesozoic. 



CHAPTEE XX 
THE MESOZOIC 

With the close of the Permian the world of animal and 
vegetable life had so changed that the line is drawn here which 
marks the end of the old order and the beginning of the new and 
separates the Paleozoic from the succeeding era, the Mes- 
ozoic, the Middle Age of geological history. Although the 
Mesozoic era is shorter than the Paleozoic, as measured by the 
thickness of their strata, yet its duration must be reckoned in 
millions of years. Its predominant life features are the culmi- 
nation and the beginning of the decline of reptiles, amphibians, 
cephalopod mollusks, and cycads, and the advent of marsupial 
mammals, birds, teleost fishes, and angiospermous plants. The 
leading events of the long ages of the era we can sketch only 
in the most summary way. 

The Mesozoic comprises three systems, the Triassic, named 
from its threefold division in Germany ; the Jurassic, which is 
well displayed in the Jura Mountains ; and the Cretaceous, which 
contains the extensive chalk (Latin, creta) deposits of Europe. 

In eastern North America the Mesozoic rocks are much less impor- 
tant than the Paleozoic, for much of this portion of the continent was 
land during the Mesozoic era, and the area of the Mesozoic rocks is 
small. In western North America, on the other hand, the strata of the 
Mesozoic and of the Cenozoic also are widely spread. The Paleo- 
zoic rocks are buried quite generally from view except where the moun- 
tain makings and continental uplifts of the Mesozoic and Cenozoic 
have allowed profound erosion to bring them to light, as in deep can- 
yons and about mountain axes. The record of many of the most impor- 
tant events in the development of the continent during the Mesozoic 
and Cenozoic eras is found in the rocks of our western states. 

308 



THE MESOZOIC 369 

THE TRIASSIC AND JURASSIC 

Eastern North America. The sedimentary record interrupted 
by the Appalachian deformation was not renewed in eastern 
North America until late in the Triassic. Hence during this 
long interval the land stood high, the coast was farther out than 
now, and over our Atlantic states geological time was recorded 
chiefly in erosion forms of hill and plain which have long since 
vanished. The area of the later Triassic rocks of this region, 
which take up again the geological record, is seen in the map 
of Figure 260. They lie on the upturned and eroded edges of 
the older rocks and occupy long troughs running for the most 
part parallel to the Atlantic coast. Evidently subsidence was in 
progress where these rocks were deposited. The eastern border 
of Appalachia was now depressed. The oldland was warping, 
and long belts of country lying parallel to the shore subsided, 
forming troughs in which thousands of feet of sediment now 
gathered. 

These Triassic rocks, which are chiefly sandstones, hold no 
marine fossils, and hence were not laid in open arms of the sea. 
But their layers are often ripple-marked, and contain many 
tracks of reptiles, imprints of raindrops, and some fossil wood, 
while an occasional bed of shale is filled with the remains of 
fishes. We may conceive, then, of the Connecticut valley and 
the larger trough to the southwest as basins gradually sinking 
at a rate perhaps no faster than that of the New Jersey coast 
to-day, and as gradually aggraded by streams from the neigh- 
boring uplands. Their broad, sandy flats were overflowed by 
wandering streams, and when subsidence gained on deposition 
shallow lakes overspread the alluvial plains. Perhaps now and 
then the basins became long, brackish estuaries, whose low shores 
were swept by the incoming tide and were in turn left bare at 
its retreat to receive the rain prints of passing showers and the 
tracks of the troops of reptiles which inhabited these valleys. 






370 THE ELEMENTS OF GEOLOGY 

The Triassic rocks are mainly red sandstones, often feldspathic, 
or arkose, with some conglomerates and shales. Considering the large 
amount of feldspathic material in these rocks, do you infer that they 
were derived from the adjacent crystalline and metamorphic rocks of 
the oldland of Appalachia, or from the sedimentary Paleozoic rocks 
which had been folded into mountains during the Appalachian defor- 
mation ? If from the former, was the drainage of the northern Appa- 
lachian mountain region then, as now, eastward and southeastward 
toward the Atlantic ? The Triassic sandstones are voluminous, measur- 
ing at least a mile in thickness, and are largely of coarse waste. What 
do you infer as to the height of the lands from which the waste was 
shed, or the direction of the oscillation which they were then under- 
going ? In the southern basins, as about Richmond, Virginia, are valu- 
able beds of coal ; what was the physical geography of these areas when 
the coal was being formed? 

Interbedded with the Triassic sandstones are contempora- 
neous lava beds which were fed from dikes. Volcanic action, 




FIG. 315. Section of Triassic Sandstones of the Connecticut Valley 
ss, sandstones ; II, lava sheets ; cc, crystalline igneous and metamorphic rocks 

which had been remarkably absent in eastern North America 
during Paleozoic times, was well-marked in connection with 
the warping now in progress. Thick intrusive' sheets have also 
been driven in among the strata, as, for example, the 'sheet of 
the Palisades of the Hudson, described on page 269. 

The present condition of the Triassic sandstones of the Connecticut 
valley is seen in Figure 315. Were the beds laid in their present atti- 
tude ? What was the nature of the deformation which they have suf- 
fered ? When did the intrusion of lava sheets take place relative to the 
deformation ? What effect have these sheets on the present topography, 
and why ? Assuming that the Triassic deformation went on more rapidly 
than denudation, what was its effect on the topography of the time ? Are 
there any of its results remaining in the topography of to-day ? Do the 



THE MESOZOIC 371 

sic areas now stand higher or lower than the surrounding country, 
id why ? How do the Triassic sandstones and shales compare in hard- 
ness with the igneous and metamorphic rocks about them ? The Jurassic 
strata are wanting over the Triassic areas and over all of eastern North 
America. Was this region laud or sea, an area of erosion or sedimenta- 
tion, during the Jurassic period ? In New Jersey, Pennsylvania, and far- 
ther southwest the lowest strata of the next period, the Cretaceous, rest 
on the eroded edges of the earlier rocks. The surface on which they lie 
is worn so even that we must believe that at the opening of the Creta- 
ceous the oldland of Appalachia, including the Triassic areas, had been 
baseleveled at least near the coast. When, therefore, did the deformation 
of the Triassic rocks occur? 

Western North America. Triassic strata infolded in the Sierra 
Nevada Mountains carry marine fossils and reach a thickness of 
nearly five thousand feet. California was then under water, and 
the site of the Sierra was a subsiding trough slowly filling with 
waste from the Great Basin land to the east. 

Over a long belt which reaches from Wyoming across Colorado into 
New Mexico no Triassic sediments are found, nor is there any evidence 
that they were ever present ; hence this area was high land suffering 
erosion during the Triassic. On each side of it, in eastern Colorado 
and about the Black Hills, in western Texas, in Utah, over the site of 
the Wasatch Mountains, and southward into Arizona over the plateaus 
trenched by the Colorado River, are large areas of Triassic rocks, sand- 
stones chiefly, with some rock salt and gypsum. Fossils are very rare 
and none of them marine. Here, then, lay broad shallow lakes often 
salt, and warped basins, in which the waste of the adjacent uplands 
gathered. To this system belong the sandstones of the Garden of the 
Gods in Colorado, which later earth movements have upturned with 
the uplifted mountain flanks. 

The Jurassic was marked with varied oscillations and wide 
changes in the outline of sea and land. 

Jurassic shales of immense thickness now metamorphosed 
into slates are found infolded into the Sierra Nevada Moun- 
tains. Hence during Jurassic times the Sierra trough continued 



872 THE ELEMENTS OF GEOLOGY 

to subside, and enormous deposits of mud were washed into it 
from the land lying to the east. Contemporaneous lava flows 
interbedded with the strata show that volcanic action accom- 
panied the downwarp, and that molten rock was driven upward 
through fissures in the crust and outspread over the sea floor in 
sheets of lava. 

The Sierra deformation. Ever since the middle of the Silu- 
rian, the Sierra trough had been sinking, though 110 doubt 
with halts and interruptions, until it contained nearly twenty- 
five thousand feet of sediment. At the close of the Jurassic it 
yielded to lateral pressure and the vast pile of strata was crum- 
pled and upheaved into towering mountains. The Mesozoic 
muds were hardened and squeezed into slates. The rocks were 
wrenched and broken, and underground waters began the work 
of filling their fissures with gold-bearing quartz, which was yet to 
wait millions of years before the arrival of man to mine it. Im- 
mense bodies of molten rock were intruded into the crust as it suf- . 
fered deformation, and these appear in the large areas of granite 
which the later denudation of the range has brought to light. 

The same movements probably uplifted the rocks of the 
Coast Eange in a chain of islands. The whole western part of 
the continent was raised and its seas and lakes were for the 
most part drained away. 

The British Isles. The Triassic strata of the British Isles are conti- 
nental, and include breccia beds of cemented talus, deposits of salt and 
gypsum, and sandstones whose rounded and polished grains are those 
of the wind-blown sands of deserts. In Triassic times the British Isles 
were part of a desert extending over much of northwestern Europe. 

THE CRETACEOUS 

The third great system of the Mesozoic includes many forma- 
tions, marine and continental, which record a long and compli- 
cated history marked by great oscillations of the crust and wide 
changes in the outlines of sea and land. 



THE MESOZOIC 373 

Early Cretaceous. In eastern North America the lowest Cretaceous 
series comprises fresh-water formations which are traced from Nantucket 
across Martha's Vineyard and Long Island, and through New Jersey 
southward into Georgia. They rest unconformably on the Triassic 
sandstones and the older rocks of the region. The Atlantic shore line 
was still farther out than now in the northern states. Again, as during 
the Triassic, a warping of the crust formed a long trough parallel to the 
coast and to the Appalachian ridges, but cut off from the sea ; and 
here the continental deposits of the early Cretaceous were laid. 

Along the Gulf of Mexico the same series was deposited under like 
conditions over the area known as the Mississippi embayment, reaching 
from Georgia northwestward into Tennessee and thence across into 
Arkansas and southward into Texas. 

In the Southwest the subsidence continued until the transgressing 
sea covered most of Mexico and Texas and extended a gulf northward 
into Kansas. In its warm and quiet waters limestones accumulated to 
a depth of from one thousand to five thousand feet in Texas, and of 
more than ten thousand feet in Mexico. Meanwhile the lowlands, where 
the Great Plains are now, received continental deposits ; coal swamps 
stretched from western Montana into British Columbia. 

The Middle Cretaceous. This was a land epoch. The early Cretaceous 
sea retired from Texas and Mexico, for its sediments are overlain 
unconformably by formations of the Upper Cretaceous. So long was 
the time gap between the two series that no species found in the one 
occurs in the other. 

The Upper Cretaceous. There now began one of the most 
remarkable events in all geological history, the great Creta- 
ceous subsidence. Its earlier warpings were recorded in conti- 
nental deposits, wide sheets of sandstone, shale, and some 
coal, which were spread from Texas to British Columbia. 
These continental deposits are overlain by a succession of marine 
formations whose vast area is shown on the map, Figure 260. We 
may infer that as the depression of the continent continued the sea 
came in far and wide over the coast lands and the plains worn 
low during the previous epochs. Upper Cretaceous formations 
show that south of New England the waters of the Atlantic 



374 



THE ELEMENTS OF GEOLOGY 



somewhat overlapped the crystalline rocks of the Piedmont Belt 
and spread their waste over the submerged coastal plain. The 
Gulf of Mexico again covered the Mississippi embayment, reach- 




FIG. 316. Hypothetical Map of Upper Cretaceous Epicontinental Seas 
Shaded areas, probable seas ; broken lines, approximate shore lines 

ing as far north as southern Illinois, and extended over Texas. 

A mediterranean sea now stretched from the Gulf to the arctic 

regions and from central 
Iowa to the eastern shore of 
the Great Basin land at about 
the longitude of Salt Lake 
City, the Colorado Mountains 
rising from it in a chain of 
islands. Along with minor 
oscillations there were laid 
in the interior sea various 
formations of sandstones, 
FIG. 317. Foraminifera from Creta- shales, and limestones, and 
ceous Chalk, Iowa. Magnified from Kansas to South Dakota 

beds of white chalk show that the clear, warm waters swarmed 

at times with foraminiferal life whose disintegrating microscopic 

shells accumulated in this rare deposit. 




THE MESOZOIC 375 

At this epoch a wide sea, interrupted by various islands, stretched 
across Eurasia from Wales and western Spain to China, and spread 
southward over much of the Sahara. To the west its waters were clear 
and on its floor the crumbled remains of foraminifers gathered in 
heavy accumulations of calcareous ooze, the white chalk of France 
and England. Sea urchins were also abundant, and sponges contributed 
their spicules to form nodules of flint. 

The Laramie. The closing stage of the Cretaceous was marked in 
North America by a slow uplift of the land. As the interior sea gradu- 
ally withdrew, the warping basins of its floor were filled with waste 
from the rising lands about them, and over this wide area there were 
spread continental deposits in fresh-water lakes like the Great Lakes of 
the present, in brackish estuaries, and in river plains, while occasional 
oscillations now and again let in the sea. There were vast marshes in 
which there accumulated the larger part of the valuable coal seams of 
the West. The Laramie is the coal-bearing series of the West, as the 
Pennsylvanian is of the eastern part of our country. 

The Rocky Mountain deformation. At the close of the Cre- 
taceous we enter upon an epoch of mountain-making far more 
extensive than any which the continent had witnessed. The 
long belt lying west of the ancient axes of the Colorado Islands 
and east of the Great Basin land had been an area of deposition 
for many ages, and in its subsiding troughs Paleozoic and Meso- 
zoic sediments had gathered to the depth of many thousand 
feet. And now from Mexico well-nigh to the Arctic Ocean this 
belt yielded to lateral pressure. The Cretaceous limestones of 
Mexico were folded into lofty mountains. A massive range was 
upfolded where the Wasatch Mountains now are, and various 
ranges of the Kockies in Colorado and other states were upridged. 
However slowly these deformations were effected they were no 
doubt accompanied by world-shaking earthquakes, and it is 
known that volcanic eruptions took place on a magnificent 
scale. Outflows of lava occurred along the Wasatch, the lacco- 
liths of the Henry Mountains (p. 271) were formed, while the 
great masses of igneous rock which constitute the cores of the 



376 THE ELEMENTS OF GEOLOGY 

Spanish Peaks (p. 271) and other western mountains were thrust 
up amid the strata. The high plateaus from which many of 
these ranges rise had not yet been uplifted, and the bases of the 
mountains probably stood near the level of the sea. 

North America was now well-nigh completed. The medi- 
terranean seas which so often had occupied the heart of the 
land were done away with, and the continent stretched unbroken 
from the foot of the Sierras on the west to the Fall "Line of the 
Atlantic coastal plain on the east. 

The Mesozoic peneplain. The immense thickness of the Mes- 
ozoic formations conveys to our minds some idea of the vast 
length of time involved in the slow progress of its successive 
ages. The same lesson is taught as plainly by the amount of 
denudation which the lands suffered during the era. 

The beginning of the Mesozoic saw a system of lofty mountain 
ranges stretching from New York into central Alabama. The 
end of this long era found here a wide peneplain crossed by 
sluggish wandering rivers and overlooked by detached hills as 
yet unreduced to the general level. The Mesozoic era was long 
enough for the Appalachian Mountains, upridged at its begin- 
ning, to have been weathered and worn away and carried grain 
by grain to the sea. The same plain extended over southern 
New England. The Taconic range, uplifted partially at least at 
the close of the Ordovician, and the block mountains of the 
Triassic, together with the pre-Cambrian mountains of ancient 
Appalachia, had now all been worn to a common level with the 
Allegheny ranges. The Mesozoic peneplain has been upwarped 
by later crustal movements and has suffered profound erosion, but 
the remnants of it which remain on the upland of southern New 
England and the even summits of the Allegheny ridges suffice 
to prove that it once existed. The age of the Mesozoic peneplain 
is determined from the fact that the lower Tertiary sediments 
were deposited on its even surface when at the close of the era 
the peneplain was depressed along its edges beneath the sea. 



THE MESOZOIC 



377 



LIFE OF THE MESOZOIC . 

Plant life of the Triassic and Jurassic. The Carboniferous 
forests of lepidodendrons and sigillarids had now vanished 
from the earth. The uplands were clothed with conifers, like 
the Araucarian pines of South America and Australia. Dense 
forests of tree ferns throve in moist regions, and canebrakes of 
horsetails of modern type, but with stems reaching four inches 
in thickness, bordered the lagoons and marshes. Cycads were 




FIG. 318. A Living Cycad of 
Australia 



FIG. 319. Stem of a Mesozoic 
Cycad 



exceedingly abundant. These gymnosperms, related to the pines 
and spruces in structure and fruiting, but palmlike in their foli- 
age, and uncoiling their long leaves after the manner of ferns, 
culminated in the Jurassic. From the view point of the bota- 
nist the Mesozoic is the Age of Cycads, and after this era they 
gradually decline to the small number of species now existing in 
tropical latitudes. 

Plant life of the Cretaceous. In the Lower Cretaceous the 
woodlands continued of much the same type as during the 
Jurassic. The forerunners now appeared of the modem dicotyls 
(plants with two seed leaves), and in the Middle Cretaceous the 
monocotyledonous group of palms came in. Palms are so like 
cycads that we may regard them as the descendants of some 
cycad type. 



378 THE ELEMENTS OF GEOLOGY 

In the Upper Cretaceous, cycads become rare. The highest 
types of flowering plants gain a complete ascendency, and 
forests of modern aspect cover the continent from the Gulf of 
Mexico to the Arctic Ocean. Among the kinds of forest trees 
whose remains are found in the continental deposits of the 
Cretaceous are the magnolia, the myrtle, the laurel, the fig, 
the tulip tree, the chestnut, the oak, beech, elm, poplar, wil- 
low, birch, and maple. Forests of Eucalyptus grew along the 
coast of New England, and palms on the Pacific shores of 
British Columbia. Sequoias of many varieties ranged far into 
northern Canada. In northern Greenland there were luxuriant 
forests of magnolias, figs, and cycads ; and a similar flora has 
been disinterred from the Cretaceous rocks of Alaska and Spitz- 
bergen. Evidently the lands within the Arctic Circle enjoyed a 
warm and genial climate, as they had done during the Paleozoic. 
Greenland had the temperature of Cuba and southern Florida, 
and the time was yet far distant when it was to be wrapped in 
glacier ice. 

Invertebrates. During the long succession of the ages of the 
Mesozoic, with their vast geographical changes, there were many 

and great changes 
in organisms. Spe- 
cies were replaced 
again and again by 
others better fitted 
to the changing en- 
vironment. During 
FIG. 320. A Jurassic Long-Tailed Crustacean 

the Lower Creta- 
ceous alone there were no less than six successive changes in 
the faunas which inhabited the limestone-making sea which 
then covered Texas. We shall disregard these changes for the 
most part in describing the life of the era, and shall confine 
our view to some of the most important advances made in the 
leading types. 




THE MESOZOIC 



379 



Stromatopora have disappeared. Protozoans and sponges are 
exceedingly abundant, and all contribute to the making of 
Mesozoic strata. Corals 
have assumed a more 
modern type. Sea 
urchins have become 
plentiful; crinoids 
abound until the Cre- 
taceous, where they 
begin their decline to 
their present humble 
station. 

Trilobites and euryp- 
terids are gone. Ten- 
footed crustaceans abound of the primitive long-tailed type 
(represented by the lobster and the crayfish), and in the Jurassic 
there appears the modern short-tailed type represented by the 
crabs. The latter type is higher hi organ- 
ization and now far more common. In 
its embryological development it passes 




FIG. 321. A Fossil Crab 





FIG. 322. Cretaceous Mollusks 
A, Ostrea (oyster) ; B, Exogyra; C, Gryphsea 

through the long-tailed stage ; connecting links in the Meso- 
zoic also indicate that the younger type is the offshoot of 
the older. 



380 



THE ELEMENTS OF GEOLOGY 



Insects evolve along diverse lines, giving rise to beetles, ants, 
bees, and flies. 

Brachiopods have dwindled greatly in the number of their 
species, while mollusks have correspondingly increased. The 
great oyster family dates from here. 

Cephalopods are now to have their day. The archaic Or- 
thoceras lingers 011 into the Triassic and becomes extinct, but a 
remarkable development is now at hand for the more highly 
organized descendants of this 
ancient line. We have noticed 
Hint in the Devonian the 





FIG. 323. Ceratites FIG. 324. An Ammonite 

A portion of the shell is removed to show 
frilling of suture 

sutures of some of the chambered shells become angled, evolv- 
ing the Goniatite type (p. 344). The sutures now become lobed 
and corrugated in Ceratites. The process was carried still farther, 
and the sutures were elaborately frilled in the great order of 
the Ammonites (Fig. 324). It was in the Jurassic that the Am- 
monites reached their height. No fossils are more abundant 
or characteristic of their age. Great banks of their shells formed 
beds of limestone in warm seas the world over. 



THE MESOZOIC 



381 




FIG. 325. Slab of Rock covered with Ammonites, a Bit of 
a Mesozoic Sea Bottom 




FIG. 326. Representative Species of Different Families 
of Ammonoids 



382 



THE ELEMENTS OF GEOLOGY 



The ammonite 'stem branched into a most luxuriant variety 
of forms. The typical form was closely coiled like a nautilus 
(Fig. 325). In others (Fig. 326) the coil was more or less open, 
or even erected into a spiral. Some were hook-shaped, and there 
were members of the order in which the shell was straight, and 
yet retained all the internal structures of its kind. At the end 
of the Mesozoic the entire tribe of ammonites became extinct. 

The Belemnite (Greek, belemnon, a dart) is a 
distinctly higher type of cephalopod which ap- 
peared in the Triassic, became numerous and 
varied in the Jurassic and Cretaceous, and died 
out early in the Tertiary. Like the squids and 
cuttlefish, of which it was the prototype, it had 
an internal calcareous shell (Fig. 327). This 
consisted of a chambered and siphuncled cone 
(Fig. 327, Ph), whose point was sheathed in a 
long solid guard (Fig. 327, R) somewhat like a 
dart. The animal carried an ink sac, and no 
doubt used it as that of the modern cuttlefish 
is used, to darken the water and make easy 
an escape from foes. Belemnites have some- 
times been sketched with fossil sepia, or india 
ink, from their own ink sacs. In the belemnites 
and their descendants, the squids and cuttle- 
FIG. 327. Internal fi s h, the cephalopods made the radical change 
' lemnite from the external to the internal shell. They 
abandoned the defensive system of warfare and boldly took up 
the offensive. No doubt, like their descendants, the belemnites 
were exceedingly active and voracious creatures. 

Fishes and amphibians. In the Triassic and Jurassic, little 
progress was made among the fishes, and the ganoid was still 
the leading type. In the Cretaceous the teleosts, or bony 
fishes (p. 349), made their appearance, while ganoids declined 
toward their present subordinate place. 



-Ph. 



THE MESOZOIC 383 

The amphibians culminated in the Triassic, some being formi- 
dable creatures as large as alligators. They were still of the 
primitive Paleozoic types (p. 364). Their pygmy descendants 
of more modern types are not found until later, salamanders 
appearing first in the Cretaceous, and frogs at the beginning of 
the Cenozoic. 

No remains of amphibians have been discovered in the Jurassic. Do 
you infer from this that there were none in existence at that time ? 

Reptiles of the Mesozoic 

The great order of Keptiles made its advent in the Permian, 
culminated in the Triassic and Jurassic, and began to decline 
in the Cretaceous. The advance from the amphibian to the 
reptile was a long forward step in the evolution of the verte- 
brates. In the reptile the vertebrate skeleton now became com- 
pletely ossified. Gills were abandoned and breathing was by 
lungs alone. The development of the individual from the egg 
to maturity was uninterrupted by any metamorphosis, such as 
that of the frog when it passes from the tadpole stage. Yet in 
advancing from the amphibian to the reptile the evolution of 
the vertebrate was far from finished. The cold-blooded, clumsy 
and sluggish, small-brained and unintelligent reptile is as far 
inferior to the higher mammals, whose day was still to come, as 
it is superior to the amphibian and the fish. 

The reptiles of the Permian, the earliest known, were much 
like lizards in form of body. Constituting a transition type 
between the amphibians on the one hand, and both the higher 
reptiles and the mammals on the other, they retained the 
archaic biconcave vertebrae of the fish and in some cases the 
persistent notochord, while some of them, the theromorphs, 
possessed characters allying them with mammals. In these the 
skull was remarkably similar to that of the carnivores, or flesh- 
eating mammals, and the teeth, unlike the teeth of any later 




384 THE ELEMENTS OF GEOLOGY 

reptiles, were divisible into incisors, canines, and molars, as are 

the teeth of mammals (Fig. 328). 

At the opening of the Mesozoic era reptiles were the most 

highly organized and powerful of any animals on. the earth. 

New ranges of continental extent were opened to them, food 

was abundant, the climate was congenial, and they now branched 

into very many diverse types 
which occupied and ruled 
all fields, the land, the air, 
and the sea. The Mesozoic 
was the Age of Reptiles. 
The ancestry of surviving 

FIG. 328. Skull of a Permian Theromorph reptilian ty p es . We will 

consider first the evolution of the few reptilian types which 
have survived to the present. 

Crocodiles, the highest of existing reptiles, are a very ancient 
order, dating back to the lower Jurassic, and traceable to earlier 
ancestral, generalized forms, from which sprang several other 
orders also. 

Turtles and tortoises are not found until the early Jurassic, 
when they already possessed the peculiar characteristics which 
set them off so sharply from other reptiles. They seem to have 
lived at first in shallow water and in swamps, and it is not until 
after the end of the Mesozoic that some of the order became 
adapted to life on the land. 

The largest of all known turtles, Archelon, whose home was the 
great interior Cretaceous sea, was fully a dozen feet in length and must 
have weighed at least two tons. The skull alone is a yard long. 

Lizards and snakes do not appear until after the close of the 
Mesozoic, although their ancestral lines may be followed back 
into the Cretaceous. 

We will now describe some of the highly specialized orders 
peculiar to the Mesozoic. 



THE MESOZOIC 



385 



Land reptiles. The dinosaurs (terrible reptiles) are an ex- 
tremely varied order which were masters of the land from 
the late Trias until the close of the Mesozoic era. Some 
were far larger than elephants, some were as small as cats ; 
some walked on all fours, some were bipedal ; some fed on the 
luxuriant tropical foliage, and others on the flesh of weaker 
reptiles. They may be classed in three divisions, the flesh- 
eating dinosaurs, the reptile-footed dinosaurs, and the leaked 
dinosaurs, the latter two divisions being herbivorous. 

The flesh-eating dinosaurs are the oldest known division of 
the order, and their characteristics are shown in Figure 329. 
As a class, reptiles are egg layers (oviparous) ; but some of 
the flesh-eating dinosaurs are known to have been viviparous, 
i.e. to have brought forth their young alive. This group was 
the longest-lived of any of the three, beginning in the Trias 
and continuing to the close of the Mesozoic era. 




FIG. 329. Ceratosaurus 
From' Animals of the Past. By the Courtesy of McClure, Phillips & Co. 

Contrast the small fore limbs, used only for grasping, with the 
powerful hind limbs on which the animal stalked about. Note the sharp 
claws and ponderous tail. The ceratosaur was fifteen feet long, but a 



386 



THE ELEMENTS OF GEOLOGY 




very similar monster, the tyrannosaur, 
reached a length of forty-seven feet and 
a standing height of twenty feet. "In 
speed, size, power, and ferocity " this rep- 
tile was "the most destructive life engine 
which has ever evolved." Its swift rush 
must have been well-nigh resistless. 

The reptile-footed dinosaurs (Sauro- 
poda) include some of the biggest 
brutes which ever trod the ground. 
One of the largest, whose remains are 
found entombed in the Jurassic rocks 
of Wyoming and Colorado, is shown 
in Figure 330. 

Note the five digits on the .hind feet, 
the quadrupedal gait, the enormous stretch 
of neck and tail, the small head aligned 
with the vertebral column. Diplodocus was 
fully sixty-five feet long and must have 
weighed about twenty tons. The thigh 
bones of the Sauropoda are the largest 
bones which ever grew. That of a genus 
allied to the Diplodocus measures six feet 
and eight inches, and the total length of 
the animal must have been not far from 
eighty feet, the largest land animal known. 

The Sauropoda became extinct 
when their haunts along the rivers 
and lakes of the western plains of 
Jurassic times were invaded by the 
Cretaceous interior sea. 

The leaked dinosaurs (Predentata) 
were distinguished by a beak sheathed 
with horn carried in front of the tooth- 
set jaw, and used, we may imagine, in 



THE MESOZOIC 



387 



stripping the leaves and twigs of trees and shrubs. We may 
notice only two of the most interesting types. 

Stegosaurus (plated reptile) takes its name from the double row of 
bony plates arranged along its back. The powerful tail was armed 
with long spines, and the thick skin was defended with irregular bits 
of bone implanted in it. The brain of the stegosaur was smaller than 
that of any land vertebrate, while in the sacrum the nerve canal was 
enlarged to ten times the capacity of the brain cavity of the skull. 
Despite their feeble wits, this well-armored family lived on through 




FIG. 331. Stegosaurus 

millions of years which intervened between their appearance, at the 
opening of the Jurassic, and the close of the Cretaceous, when they 
became extinct. 

A less stupid brute than the stegosaur was Triceratops, the dinosaur 
of the three horns, one horn carried on the nose, and a massive pair 
set over the eyes (Fig. 332). Note the enormous wedge-shaped skull, 
with its sharp beak, and the hood behind resembling a fireman's helmet. 
Triceratops was fully twenty-five feet long, and of twice the bulk of an 
elephant. The family appeared in the Upper Cretaceous and became 
extinct at its close. Their bones are found buried in the fresh-water 
deposits of the time from Colorado to Montana and eastward to the 
Dakotas. 



THE MESOZOIC 



389 



Marine reptiles. In the ocean, reptiles occupied the place 
now held by the aquatic mammals, such as whales and dol- 
phins, and their form and structure were similarly modified to 
suit their environment. In the Ichthyosaurus (fish reptile), for 




FIG. 333. Ichthyosaurus 

example, the body was fishlike in form, with short neck and 
large, pointed head (Fig. 333). 

A powerful tail, whose flukes were set vertical, and the lower one of 
which was vertebrated, served as propeller, while a large dorsal fin was 
developed as a cutwater. The primitive biconcave vertebrae of the fish 




FIG. 334. Plesiosaurus 

and of the early land vertebrates were retained, and the limbs degen- 
erated into short paddles. The skin of the ichthyosaur was smooth 
like that of a whale, and its food was largely fish and cephalopods, as 
the fossil contents of its stomach prove. 

These sea monsters disported along the Pacific shore over 
northern California in Triassic times, and the bones of immense 
members of the family occur in the Jurassic strata of Wyoming. 



390 



THE ELEMENTS OF GEOLOGY 



Like whales and seals, the ichthyosaurs were descended from 

land vertebrates which had become adapted to a marine habitat. 

Plesiosaurs were another order which ranged throughout 

the Mesozoic. Descended from small amphibious animals, they 

later included great 
marine reptiles, 
characterized in the 
typical genus by 
long neck, snakelike 
head, and immense 
paddles. They swam 
in the Cretaceous 
interior sea of west- 



ern North America. 
Mosasaurs belong 
to the same order 
as do snakes and 




ancestral line of 
land reptiles. These 
snakelike creatures 

which measured 
as much as forty- 
five feet in length 

abounded in the 
Cretaceous seas. 

They had large conical teeth, and their limbs had become 
stout paddles. 



FIG. 335. Restoration of a Mosasaur 

From Animals of the Past. By the courtesy of 
McClui-e, Phillips & Co. 



The lower jaw of the mosasaur was jointed ; the quadrate bone, 
which in all reptiles connects the bone of the lower jaw with the skull, 
was movable, and as in snakes the lower jaw could be used in thrust- 
ing prey down the throat. The family became extinct at the end of the 
Mesozoic, and left no descendants. One may imitate the movement of 



THE MESOZOIC 



391 



the lower jaw of the mosasaur by extending the arms, clasping the 
hands, and bending the elbows. 

Flying reptiles. The atmosphere, which had hitherto been 
tenanted only by insects, was first conquered by the verte- 

brates in the 
Meso'zoic. Pter- 
osaurs, winged 
reptiles, whose 
whole organism 
was adapted for 
flight through the 
air, appeared in 
the Jurassic and passed off the stage of existence before the 
end of the Cretaceous. The bones were hollow, as are those of 
birds. The sternum, or breastbone, was given a keel for the 
attachment of the wing muscles. The fifth finger, prodigiously 




FIG. 336. Restoration of a Pterosaur 




FIG. 337. Skeletons of the Pterosaur Oruithostoma, A, 
and of the Condor, B 

After Lucas 

lengthened, was turned backward to support a membrane which 
was attached to the body and extended to the base of the tail 
The other fingers were free, and armed with sharp and delicate 
claws, as shown in Figures 336 and 337. 



392 



THE ELEMENTS OF GEOLOGY 



n 



These " dragons of the air " varied greatly in size ; some were as 
small as sparrows, while others surpassed in stretch of wing the largest 
birds of the present day. They may be divided into two groups. The 
earliest group comprises genera with jaws set with teeth, and with long 

tails sometimes provided 
with a rudderlike ex- 
pansion at .the end. In 
their successors of the 
later group the tail had 
become short, and in 
some of the genera the 
teeth had disappeared. 
Among the latest of the 
flying reptiles was Orni- 
thosloma (bird beak), the 
largest creature which 
ever flew, and whose re- 
mains are imbedded in 
the offshore deposits of 
the Cretaceous sea which 
held sway over our west- 
ern plains. Ornithosto- 
ma's spread of wings 
was twenty feet. Its 
bones were a marvel of 
lightness, the entire 
skeleton, even in its pet- 
rified condition, not 
weighing more than five 
or six pounds. The 
sharp beak, a yard long, 
was toothless and bird- 
like, as its name sug- 




IV H 



Ifl 



FIG. 338. Archseopteryx 



gests. 



Birds. The earliest-known birds are found in the Jurassic, 
and during the remainder of the Mesozoic they contended with 
the flying reptiles for the empire of the air. The first feathered 



THE MESOZOIC 

creatures were very different from the birds of to-day. Their 
characteristics prove them an offshoot of the dinosaur line 
of reptiles. Archceopteryx (ancient bird) (Fig. 338) exhibits a 
strange mingling of bird and reptile. like birds, it was fledged 
with perfect feathers, at least on wings and tail, but it retained 
the teeth of the reptile, and its long tail was vertebrated, a pair 
of feathers springing from each joint. Three fingers were free 
and clawed. There were quill feathers on the legs. This and 
the fact that young birds have sprouting quills on the le^ 
from knee to tail suggest a four-winged ancestor capable of 
gliding leap from trees, whose feathers were developing from 
reptilian scales. 

Mammals. So far as the entries upon the geological record 
show, mammals made their advent in a very humble way dur- 
ing the Trias. These earliest of vertebrates which suckle their 
young were no bigger than young 
kittens, and their strong affinities 
with the theromorphs suggest that 
their ancestors are to be found 

among some generalized types of FlG - 3 ^9. Jawbone of a 
,v j .'-I Jurassic Mammal 

that order of reptiles. 

During the long ages of the Mesozoic, mammals continued 
small and few, and were completely dominated by the reptiles. 
Their remains are exceedingly rare, and consist of minute scat- 
tered teeth, with an occasional detached jaw, which prove 
them to have been flesh or insect eaters. In the same way 
their affinities are seen to be with the lowest of mammals, 
the monotremes and marsupials. The monotremes, such as 
the duckbill mole and the spiny ant-eater of Australia, reproduce 
by means of eggs resembling those of reptiles ; the marsupials, 
such as the opossum and the kangaroo, bring forth their young 
alive, but in a very immature condition, and carry them for 
some time after birth in the marsupium, a pouch on the ventral 
side of the body. 




CHAPTEK XXI 
THE TERTIARY 

The Cenozoic era. The last stages of the Cretaceous are 
marked by a decadence of the reptiles. By the end of that 
period the reptilian forms characteristic of the time had become 
extinct one after another, leaving to represent the class only 
the types of reptiles which continue to modern times. The day 
of the ammonite and the belemnite also now drew to a close, 
and only a few of these cephalopods were left to survive the 
period. It is therefore at the close of the Cretaceous that the 
line is drawn which marks the end of the Middle Age of geol- 
ogy and the beginning of the Cenozoic era, the era of modern 
life, the Age of Mammals. 

In place of the giant reptiles, mammals now become masters 
of the land, appearing first in generalized types which, during 
the long ages of the era, gradually evolve to higher forms, more 
specialized and ever more closely resembling the mammals of 
the present. In the atmosphere the flying dragons of the Meso- 
zoic give place to birds and bats. In the sea, whales, sharks, 
and teleost fishes of modern types rule in the stead of huge 
swimming reptiles. The lower vertebrates, the invertebrates of 
land and sea, and the plants of field and forest take on a modern 
aspect, and differ little more from those of to-day than the 
plants and animals of different countries now differ from one 
another. From the beginning of the Cenozoic era until now 
there is a steadily increasing number of species of animals and 
plants which have continued to exist to the present time. 

The Cenozoic era comprises two divisions, the Tertiary 
period and the Quaternary period. 

394 



THE TERTIARY 395 

In the early days of geology the formations of the entire geological 
record, so far as it was then known, were divided into three groups, 
the Primary, the Secondary (now known as the Mesozoic), and the Ter- 
tiary. When the third group was subdivided into two systems, the term 
Tertiary was retained for the first system of the two, while the term 
Quaternary was used to designate the second. 

Divisions of the Tertiary. The formations of the Tertiary 
are grouped in three divisions, the Pliocene (more recent), the 
Miocene (less recent), and the Eocene (the dawn of the recent). 

Each of these epochs is long and complex. Their various sub- 
divisions are distinguished each by its own peculiar organisms, 
and the changes of physical geography recorded in their strata. 
In the rapid view which we are compelled to take we can note 
only a few of the most conspicuous events of the period. 

Physical geography of the Tertiary in eastern North America. 
The Tertiary rocks of eastern North America are marine de- 
posits and occupy the coastal lowlands of the Atlantic and Gulf 
states (Fig. 260). In New England, Tertiary beds occur on the 
island of Martha's Vineyard, but not on the mainland ; hence 
the shore line here stood somewhat farther out than now. From 
New Jersey southward the earliest Tertiary sands and clays, still 
unconsolidated, leave only a narrow strip of the edge of the 
Cretaceous between them and the Triassic and crystalline rocks 
of the Piedmont oldland ; hence the Atlantic shore here stood 
farther in than now, and at the beginning of the period the 
present coastal plain was continental delta. A broad belt of 
Tertiary sea-laid limestones, sandstones, and shales surrounds 
the Gulf of Mexico and extends northward up the Mississippi 
embayment to the mouth of the Ohio Eiver ; hence the Gulf 
was then larger than at present, and its waters reached in a 
broad bay far up the Mississippi valley. 

Along the Atlantic coast the Mesozoic peneplain may be 
traced shoreward to where it disappears from view beneath 
an unconformable cover of early Tertiary marine strata. The 



396 THE ELEMENTS OF GEOLOGY 

beginning of the Tertiary was therefore marked by a subsidence. 
The wide erosion surface which at the close of the Mesozoic 
lay near sea level where the Appalachian Mountains and their 
neighboring plateaus and uplands now stand was lowered gently 
along its seaward edge beneath the Tertiary Atlantic to receive 
a cover of its sediments. 

As the period progressed slight oscillations occurred from 
time to time. Strips of coastal plain were added to the land, 
and as early as the close of the Miocene the shore lines of the 
Atlantic and Gulf states had reached well-nigh their present 
place. Louisiana and Florida were the last areas to emerge 
wholly from the sea, Florida being formed by a broad trans- 
verse upwarp of the continental delta at the opening of the 
Miocene, forming first an island, which afterwards was joined to 
the mainland. 

The Pacific coast. Tertiary deposits with marine fossils 
occur along the western foothills of the Sierra Nevadas, and 
are crumpled among the mountain masses of the Coast Ranges ; 
it is hence inferred that the Great Valley of California was 
then a border sea, separated from the ocean by a chain of 
mountainous islands which were upridged into the Coast Ranges 
at a still later time. Tertiary marine strata are spread over 
the lower Columbia valley and that of Puget Sound, showing 
that the Pacific came in broadly there. 

The interior of the western United States. The closing stages 
of the Mesozoic were marked, as we have seen, by the up- 
heaval of the Rocky Mountains and other western ranges. The 
bases of the mountains are now skirted by widespread Tertiary 
deposits, winch form the highest strata of the lofty plateaus from 
the level of whose summits the mountains rise. Like the re- 
cent alluvium of the Great Valley of California (p. 101), these 
deposits imply low-lying lands when they were laid, and there- 
fore at that time the mountains rose from near sea level. 
But the height ato which the Tertiary strata now stand five 




397 



398 THE ELEMENTS OF GEOLOGY 

thousand feet above the sea at Denver, and twice that height in 
the plateaus of southern Utah proves that the plateaus of 
which the Tertiary strata form a part have been uplifted during 
the Cenozoic. During their uplift, warping formed extensive 
basins both east and west of the Rockies, and in these basins 
stream-swept and lake-laid waste gathered to depths of hun- 
dreds and thousands of feet, as it is accumulating at present in 
the Great Valley of California and on the river plains of Turkes- 
tan (p. 103). The Tertiary river deposits of the High Plains 
have already been described (p. 100). How widespread are 
these ancient river plains and beds of fresh-water lakes may be 
seen in the map of Figure 260. 

The Bad Lands. In several of the western states large areas of Ter- 
tiary fresh-water deposits have been dissected to a maze of hills whose 
steep sides are cut with innumerable ravines. The deposits of these 
ancient river plains and lake beds are little cemented and because of 
the dryness of the climate are unprotected by vegetation ; hence they 
are easily carved by the wet- weather rills of scanty and infrequent rains. 
These waterless, rugged surfaces were named by the early French 
explorers the Bad Lands because they were found so difficult to traverse. 
The strata of the Bad Lands contain vast numbers of the remains of the 
animals of Tertiary times, and the large amount of barren surface 
exposed to view makes search for fossils easy and fruitful. These 
desolate tracts are therefore frequently visited by scientific collecting 
expeditions. 

Mountain making in the Tertiary. The Tertiary period 
included epochs when the earth's crust was singularly unquiet. 
From time to time on all the continents subterranean forces 
gathered head, and the crust was bent and broken and upridged 
in lofty mountains. 

The Sierra Nevada range was formed, as we have seen, by 
strata crumpling at the end of the Jurassic. But since that 
remote time the upfolded mountains had been worn to plains 
and hilly uplands, the remnants of whose uplifted erosion 



THE TERTIARY 399 

surfaces may now be traced along the western mountain slopes. 
Beginning late in the Tertiary, the region was again affected by 
mountain-making movements. A series of displacements along 
a profound fault on the eastern side tilted the enormous earth 
block of the Sierras to the west, lifting its eastern edge to form 
the lofty crest and giving to the range a steep eastern front and 
a gentle descent toward the Pacific. 

The Coast Ranges also have had a complex history with many vicis- 
situdes. The earliest foldings of their strata belong to the close of the 
Jurassic, but it was not until the end of the Miocene that the line of 
mountainous islands and the heavy sediments which had been deposited 
on their submerged flanks were crushed into a continuous mountain 
chain. Thick Pliocene beds upon their sides prove that they were 
depressed to near sea level during the later Tertiary. At the close of 
the Pliocene the Coast Ranges rose along with the upheaval of the 
Sierra, and their gradual uplift has continued to the present time. 

The numerous north-south ranges of the Great Basin and the Mount 
Saint Elias range of Alaska were also uptilted during the Tertiary. 

During the Tertiary period many of the loftiest mountains 
of the earth the Alps, the Apennines, the Pyrenees, the 
Atlas, the Caucasus, and the Himalayas received the uplift 
to which they owe most of their colossal bulk and height, as 
portions of the Tertiary sea beds now found high upon their 
flanks attest. In the Himalayas, Tertiary marine limestones 
occur sixteen thousand five hundred feet above sea level. 

Volcanic activity in the Tertiary. The vast deformations of 
the Tertiary were accompanied on a corresponding scale by out- 
pourings of lava, the outburst of volcanoes, and the intrusion of 
molten masses within the crust. In the Sierra Nevadas the 
Miocene river gravels of the valleys of the western slope, with 
their placer deposits of gold, were buried beneath streams of 
lava and beds of tuff (Fig. 258). Volcanoes broke forth along 
the Eocky Mountains and on the plateaus of Utah, New Mexico, 
and Arizona. 



400 



THE ELEMENTS OF GEOLOGY 



Mount Shasta and the immense volcanic piles of the Cascades 
date from this period. The mountain basin of the Yellowstone 
Park was filled to a depth of several thousand feet with tuffs 
and lavas, the oldest dating as far back as the beginning of the 
Tertiary. Crandall volcano (p. 263) was reared in the Miocene 
and the latest eruptions of the Park are far more recent. 

The Columbia and Snake River lavas. Still more impor- 
tant is the plateau of lava, more than two hundred thousand 
square miles in area, extending from the Yellowstone Park to the 

Cascade Mountains, 

which has been built 
from Miocene times to 
the present. 

Over this plateau, 
which occupies large por- 
tions of Idaho, Washing- 
ton, and Oregon, and 
extends into northern 
California and Nevada, 
the country rock is ba- 
saltic lava. For thousands of square miles the surface is a lava plain 
which meets the boundary mountains as a lake or sea meets a rugged 
and deeply indented coast. The floods of molten rock spread up the 
mountain valleys for a score of miles and more, the intervening spurs 
rising above the lava like long peninsulas, while here and there an 
isolated peak was left to tower above the inundation like an island 
off a submerged shore. 

The rivers which drain the plateau the Snake, the Columbia, and 
their tributaries have deeply trenched it, yet their canyons, which reach 
the depth of several thousand feet, have not been worn to the base of the 
lava except near the margin and where they cut the summits of mountains 
drowned beneath the flood. Here and there the plateau has been 
deformed. It lias been upbent into great folds, and broken into immense 
blocks of bedded lava, forming mountain ranges, which run parallel 
with the Pacific coast line. On the edges of these tilted blocks the 
thickness of the lava is seen to be fully five thousand feet. The plateau 




FIG. 341. Lava Plateau with Lava Domes 
in the Distance, Idaho 



THE TERTIARY 401 

has been built, like that of Iceland (p. 242), of innumerable overlapping 
sheets of lava. On the canyon walls they weather back in horizontal 
terraces and long talus slopes. One may distinguish each successive 
flow by its dense central portion, often jointed with large vertical col- 
umns, and the upper portion with its mass of confused irregular columns 
and scoriaceous surface. The average thickness of the flows seems to 
be about seventy-five feet. 

The plateau was long in building. Between the layers are found in 
places old soil beds and forest grounds and the sediments of lakes. 
Hence the interval between the flows in any locality was sometimes 
long enough for clays to gather in the lakes which filled depressions in 
the surface. Again and again the surface of the black basalt was 
reddened by oxidation and decayed to soil, and forests had time to grow 
upon it before the succeeding inundation sealed the sediments and soils 
away beneath a sheet of stone. Near the edges of the lava plain, rivers 
from the surrounding mountains spread sheets of sand and gravel on 
the surface of one flow after another. These pervious sands, interbedded 
with the lava, become the aquifers of artesian wells. 

In places the lavas rest on extensive lake deposits, one thousand feet 
deep, and Miocene in age as their fossils prove. It is to the middle Ter- 
tiary, then, that the earliest flows and the largest bulk of the great inun- 
dation belong. So ancient are the latest floods in the Columbia basin 
that they have weathered to a residual yellow clay from thirty to sixty 
i feet in depth and marvelously rich in the mineral substances on which 
plants feed. 

In the Snake River valley the latest lavas are much younger. Their 
surfaces are so fresh and imdecayed that here the effusive eruptions may 
well have continued to within the period of human history. Low lava 
domes like those of Iceland mark where last the basalt outwelled and 
spread far and wide before it chilled (Fig. 341). In places small mounds 
of scoria show that the eruptions were accompanied to a slight degree 
by explosions of steam. So fluid was this superheated lava that recent 
flows have been traced for more than fifty miles. 

The rocks underlying the Columbia lavas, where exposed to view, 
[ are seen to be cut by numerous great dikes of dense basalt, which mark 
the fissures through which the molten rock rose to the surface. 

The Tertiary included times of widespread and intense vol- 
canic action in other continents as well as in North America, 



402 



THE ELEMENTS OF GEOLOGY 



In Europe, Vesuvius (Fig. 231) and Etna began their career as 
submarine volcanoes in connection with earth movements which 
finally lifted Pliocene deposits in Sicily to their present height, 
four thousand feet above the sea. Volcanoes broke forth in central 
France and southern Germany, in Hungary and the Carpathians. 
Innumerable fissures opened in the crust from the north of Ire- 
land and the western islands of Scotland to the Faroes, Iceland, 
and even to arctic Greenland ; and here great plateaus were built 

of flows of basalt similar 
to that of the Columbia 
River. In India, at the 
opening of the Tertiary, 
there had been an out- 
welling of basalt, flood- 
ing to a depth of thou- 
sands of feet two hundred 
thousand square miles of 
the northwestern part of 
the peninsula (Fig. 342), 
and similar inundations 
of lava occurred where 
are now the table-lands 
of Abyssinia. From the 
middle Tertiary on, Asia 
Minor, Arabia, and Persia 
were the scenes of volcanic action. In Palestine the rise of the 
uplands of Judea at the close of the Eocene, and the down- 
faulting of the Jordan valley (p. 221) were followed by vol- 
canic outbursts. In comparison with the middle Tertiary, the 
present is a time of volcanic inactivity and repose. 

Erosion of Tertiary mountains and plateaus. The mountains 
and plateaus built at various times during the Tertiary and at 
its commencement have been profoundly carved by erosive 
agents. The Sierra Nevada Mountains have been dissected on 




FIG. 342. Map showing the Lava Sheet 
(shaded area) of Western India 



THE TERTIARY 403 

the western slope by such canyons as those of King's River and 
the Yosemite. Six miles of strata have been denuded from parts 
of the Wasatch Mountains since their rise at the beginning of 
the era. From the Colorado plateaus, whose uplift dates from the 
same time, there have been stripped off ten thousand feet of 
strata over thousands of square miles, and the colossal canyon 
of the Colorado has been cut after this great denudation had 
been mostly accomplished (Fig. 130). 

On the eastern side of the continent, as we have seen, a 
broad peneplain had been developed by the close of the Creta- 
ceous.* The remnants of this old erosion surface are now found 
upwarped to various heights in different portions of its area. 
In southern New England it now stands fifteen hundred feet 
above the sea in western Massachusetts, declining thence south- 
ward and eastward to sea level at the coast. In southwestern 
Virginia it has been lifted to four thousand feet above the sea. 
Manifestly this upwarp occurred since the peneplain was formed ; 
it is later than the Mesozoic, and the vast dissection which the 
peneplain has suffered since its uplift must belong to the suc- 
cessive cycles of Cenozoic time. 

Revived by the uplift, the streams of the area trenched it as 
deeply as its elevation permitted, and reaching grade, opened up 
wide valleys and new peneplains in the softer rocks. The Con- 
necticut valley is Tertiary in age, and in the weak Triassic 
sandstones (p. 370) has been widened in places to fifteen miles. 
Dating from the same time are the valleys of the Hudson, the 
Susquehanna, the Delaware, the Potomac, and the Shenandoah. 

In Pennsylvania and the states lying to the south the Meso- 
zoic peneplain lies along the summits of the mountain ridges. 
On the surface of this ancient plain, Tertiary erosion etched out 
the beautifully regular pattern of the Allegheny mountain 
ridges and their intervening valleys. The weaker strata of the 
long, regular folds were eroded into longitudinal valleys, while 
the hard Paleozoic sandstones, such as the Medina (p. 335) 



404 



THE ELEMENTS OF GEOLOGY 



and the Pocono (p. 350), were left in relief as bold mountain 
walls whose even crests rise to the common level of the ancient 
plain. From Virginia far into Alabama the great Appalachian 
valley was opened to a width in places of fifty miles and more, 
along a belt of intensely folded and faulted strata where once 
was the heart of the Appalachian Mountains. In Figure 70 




FIG. 343. Diagram of the Allegheny Mountains, Pennsylvania 
From Davis' Elementary Physical Geography 

the summit of the Cumberland plateau (db) marks the level 
of the Mesozoic peneplain, while the lower erosion levels are 
Tertiary and Quaternary in age. 

LIFE OF THE TERTIARY PERIOD 

Vegetation and climate. The highest plants in structure, the 
dicotyls (such as our deciduous forest trees) and the monocotyls 
(represented by the palms), were introduced during the Creta- 
ceous. The vegetable kingdom reached its culmination before 
the animal kingdom, and if the dividing line between the Meso- 
zoic and the Cenozoic were drawn according to the progress of 









THE TERTIARY 405 

plant life, the Cretaceous instead of the Tertiary would be made 
the opening period of the modern era. 

The plants of the Tertiary belonged, for the most part, to gen- 
era now living; but their distribution was very different from 
that of the flora of to-day. In the earlier Tertiary, palms flour- 
ished over northern Europe, and in the northwestern United 
States grew the magnolia and laurel, along with the walnut, 
oak, and elm. Even in northern Greenland and in Spitz- 
bergen there were lakes covered with water lilies and sur- 
rounded by forests of maples, poplars, limes, the cypress of our 
southern states, and noble sequoias similar to the " big trees " 
and redwoods of California. A warm climate like that of the 
Mesozoic, therefore, prevailed over North America and Europe, 
extending far toward the pole. In the later Tertiary the climate 
gradually became cooler. Palms disappeared from Europe, and 
everywhere the aspect of forests and open lands became more 
like that of to-day. Grasses became abundant, furnishing a new 
ood for herbivorous animals. 

Animal life of the Tertiary. Little needs to be said of the 
Tertiary invertebrates, so nearly were they like the invertebrates 
of the present. Even in the Eocene, about five per cent of 
marine shells were of species still living, and in the Pliocene 
the proportion had risen to more than one half. 

Fishes were of modern types. Teleosts were now abundant. 
The ocean teemed with sharks, some of them being voracious 
monsters seventy-five feet and even more in length, with a gape 
of jaw of six feet, as estimated by the size of their enormous 
sharp-edged teeth. 

Snakes are found for the first time in the early Tertiary. 
These limbless reptiles, evolved by degeneration from lizardlike 
ancestors, appeared in nonpoisonous types scarcely to be dis- 
tinguished from those of the present day. 

Mammals of the early Tertiary. The fossils of continental 
deposits of the earliest Eocene show that a marked advance had 



406 THE ELEMENTS OF GEOLOGY 

now been made in the evolution of the Mammalia. The higher 
mammals had appeared, and henceforth the lower mammals 
the monotremes and the marsupials are reduced to a subordi- 
nate place. 

These first true mammals were archaic and generalized in 
structure. Their feet were of the primitive type, with five toes 
of about equal length. They were also plantigrades, that is, 
they touched the ground with the sole of the entire foot from 
toe to heel. No foot had yet become adapted to swift running 
by a decrease in the number of digits and by lifting the heel and 




FIG. 344. Phenacodus 

sole so that only the toes touch the ground, a tread called 
digitigrade. Nor was there yet any foot like that of the cats, 
with sharp retractile claws adapted to seizing and tearing the 
prey. The forearm and the lower leg each had still two separate 
bones (ulna and radius, fibula and tibia), neither pair having 
been replaced with a single strong bone, as in the leg of the horse. 
The teeth also were primitive in type and of full number. The 
complex heavy grinders of the horse and elephant, the sharp cut- 
ting teeth of the carnivores, and the chopping teeth of the grass 
eaters were all still to come. 

Phenacodus is a characteristic genus of the early Eocene, whose 
species varied in size from that of a bulldog to that of an animal a little 



THE TERTIARY 407 

larger than a sheep. Its feet were primitive, and their five toes bore nails 
intermediate in form between a claw and a hoof. The archaic type of 
teeth indicates that the animal was omnivorous in diet. A cast of the 
brain cavity shows that, like its associates of the time, its brain was 
extremely small and nearly smooth, having little more than traces of 
convolutions. 

The long ages of the Eocene and the following epochs of the 
Tertiary were times of comparatively rapid evolution among the 
Mammalia. The earliest forms evolved along diverging lines 
toward the various specialized types of hoofed mammals, rodents, 
carnivores, proboscidians, the primates, and the other mammalian 
orders as we know them now. We must describe the Tertiary 
mammals very briefly, tracing the lines of descent of only a few 
of the more familiar mammals of the present. 

The horse. The pedigree of the horse runs back into the 
early Eocene through many genera and species to a five-toed, 1 
short-legged ancestor little bigger than a cat. Its descendants 
gradually increased in stature and became better and better 
adapted to swift running to escape their foes. The leg became 
longer, and only the tip of the toes struck the ground, The 
middle toe (digit number three), originally the longest of the 
five, steadily enlarged, while the remaining digits dwindled and 
disappeared. The inner digit, corresponding to the great toe and 
thumb, was the first to go. Next number five, the little finger, 
was also dropped. By the end of the Eocene a three-toed genus 
of the horse family had appeared, as large as a sheep. The hoof 
of digit number three now supported most of the weight, but 
the slender hoofs of digits two and four were still serviceable. 
In the Miocene the stature of the ancestors of the horse increased 
to that of a pony. The feet were still three-toed, but the side 
hoofs were now mere dewclaws and scarcely touched the ground. 
The evolution of the family was completed in the Pliocene. 

1 Or, more accurately, with four perfect toes and a rudimentary fifth corre- 
sponding to the thumb. 



408 



THE ELEMENTS OF GEOLOGY 



Equus 
Pleistocene 

and 
Recent 



Pliohippus 
Pliocene 



Protohippus 

Lower 
Pliocene 



Miohippus 
Miocene 



Mesohippus 
Lower 
Miocene 



Orohippus 
Eocene 



ABC 
FIG. 345. Development of Forefoot 
(A), the Forearm (E), and Molar 
(C), of the Horse Family 




The middle toe was enlarged still 
more, the side toes were dropped, 
and the palm and foot bones 
which supported them were re- 
duced to splints. 

While these changes were in 
progress the radius and ulna of 
the fore limb became consoli- 
dated to a single bone ; and in 
the hind limb the fibula dwindled 
to a splint, while the tibia was 
correspondingly enlarged. The 
molars also gradually lengthened, 
and became more and more com- 
plex on their grinding surface ; 
the neck became longer ; the 
brain steadily increased in size 
and its convolutions became 
more abundant. The evolution 
of the horse has made for greater 
fleetness and intelligence. 

The rhinoceros and tapir. 
These animals, which are grouped 
with the horse among the odd- 
toed (perissodactyl) mammals, are 
now verging toward extinction. 
In the rhinoceros, evolution 
seems to have taken the opposite 
course from that of the horse. 
As the animal increased in size 
it became more clumsy, its limbs 
became shorter and more mas- 
sive, and, perhaps because of its 
great weight, the number of digits 



THE TERTIARY 409 

were not reduced below the number three. Like other large 
herbivores, the rhinoceros, too slow to escape its enemies by 
flight, learned to withstand them. It developed as its means 
of defense a nasal horn. 

Peculiar offshoots of the line appeared at various times in the Ter- 
tiary. A rhinoceros, semiaquatic in habits, with curved tusks, resembling 
in aspect the hippopotamus, lived along the water courses of the plains 
east of the Rockies, and its bones are now found by the thousands in 
the Miocene of Kansas. Another developed along a line parallel to 
that of the horse, and herds of these light-limbed and swift-footed run- 
ning rhinoceroses ranged the Great Plains from the Dakotas southward. 




FIG. 346. A Tertiary Mastodon 

The tapirs are an ancient family which has changed but 
little since it separated from the other perissodactyl stocks in 
the early Tertiary. At present, tapirs are found only in South 
America and southern Asia, a remarkable distribution which 
we could not explain were it not that the geological record 
shows that during Tertiary times tapirs ranged throughout the 
northern hemisphere, making their way to South America late 
in that period. During the Pleistocene they became extinct over 
all the intervening lands between the widely separated regions 
where now they live. The geographic distribution of animals, 
as well as their relationships and origins, can be understood 
only through a study of their geological history. 



410 



THE ELEMENTS OF GEOLOGY 




The proboscidians. This unique order of hoofed mammals, of 
which the elephant is the sole survivor, began, so far as known, 
in the Eocene, in Egypt, with a piglike ancestor the size of a 
small horse, with cheek teeth like the Mastodon's, but want- 
ing both trunk and tusks. A proboscidian came next with 

four short tusks, and in the Miocene 
there followed a Mastodon (Eig. 346) 
armed with two pairs of long, straight 
tusks on which rested a flexible pro- 
boscis. 

The Dinothere was a curious offshoot 
of the line, which developed in the Mio- 
cene in Europe. In this immense pro- 
boscidian, whose skull was three feet 
long, the upper pair of tusks had disap- 
peared, and those of the lower jaw were 
bent down with a backward curve in 
FIG. 347. Head of Dinothere walrus fashion. 

In the true elephants, which do not appear until near the close 
of the Tertiary, the lower jaw loses its tusks and the grinding 
teeth become exceedingly complex in structure. The grinding 

teeth of the mastodon 
had long roots and low 
crowns crossed by four 
or five peaked enameled 
ridges. In the teeth of 
the true elephants the 
crown has become deep, 
and the ridges of enamel 
have changed to numerous upright, platelike folds, their inter- 
spaces filled with cement. The two genera Mastodon and 
Elephant are connected by species whose teeth are interme- 
diate in pattern. The proboscidians culminated in the Pliocene, 
when some of the giant elephants reached a height of fourteen feet. 




FIG. 348. Crown of Mastodon Tooth 



THE TERTIARY 



411 



The artiodactyls comprise the hoofed Mammalia which have 
an even number of toes, such as cattle, sheep, and swine. Like 
the perissodactyls, 
they are descended 
from the primitive 
five-toed planti- 
grade mammals of 
the lowest Eocene. 
In their evolution, 
digit number one 
was first dropped, 
and the middle pair 
became larger and more massive, while the side digits, numbers 
two and five, became shorter, weaker, and less serviceable. The 




FIG. 349. Tooth of an Extinct Elephant, 
the Mammoth 





m\J t/JV 
A BCD 

FIG. 350. Evolution of the Artiodactyl Foot, illustrated by 

Existing Families 
A, pig; B, roebuck; C, sheep; D, camel 

four-toed artiodactyls culminated in the Tertiary ; at present 
they are represented only by the hippopotamus and the hog. 



412 THE ELEMENTS OF GEOLOGY 

Along the main line of the evolution of the artiodactyls the 
side toes, digits two and five, disappeared, leaving as proof that 
they once existed the corresponding bones of palm and sole as 
splints. The two-toed artiodactyls, such as the camels, deer, 
cattle, and sheep, are now the leading types of the herbivores. 
Swine and peccaries are two branches of a common stock, 
the first developing in the Old World and the second in the 
New. In the Miocene a noticeable offshoot of the line was a 
gigantic piglike brute, a root eater, with a skull a yard in length, 
whose remains are now found in Colorado and South Dakota. 

Camels and llamas. The line of camels and llamas developed in 
North America, where the successive changes from an early Eocene 
ancestor, no larger than a rabbit, are traced step by step to the present 
forms, as clearly as is the evolution of the horse. In the late Miocene 
some of the ancestral forms migrated to the Old World by way of a land 
connection where Bering Strait now is, and there gave rise to the camels 
and dromedaries. Others migrated into South America, which had now 
been connected with our own continent, and these developed into the 
llamas and guanacos, while those of the race which remained in North 
America became extinct during the Pleistocene. 

Some peculiar branches of the camel stem appeared in North America. 
In the Pliocene arose a llama with the long neck and limbs of a giraffe, 
whose food was cropped from the leaves and branches of trees. Far 
more generalized in structure was the Oreodon, an animal related to 
the camels, but with distinct affinities also with other lines, such as 
those of the hog and deer. These curious creatures were much like the 
peccary in appearance, except for their long tails. In the middle Eocene 
they roamed in vast herds from Oregon to Kansas and Nebraska. 

The ruminants. This division of the artiodactyls includes 
antelopes, deer, oxen, bison, sheep, and goats, all of which be- 
long to a common stock which took its rise in Europe in the upper 
Eocene from ancestral forms akin to those of the camels. In 
the Miocene the evolution of the two-toed artiodactyl foot was 
well-nigh completed. Bonelike growths appeared on the head, 
and the two groups of the ruminants became specialized, the 



THE TERTIARY 413 

deer with bony antlers, shed and renewed each year, and the 
ruminants with hollow horns, whose two bony knobs upon 
the skull are covered with permanent, pointed, horny sheaths. 

The ruminants evolved in the Old World, and it was not until the 
later Miocene that the ancestors of the antelope and of some deer found 
their way to North America. Mountain sheep and goats, the bison and 
most of the deer, did not arrive until after the close of the Tertiary, 
and sheep and oxen were introduced by man. 

The hoofed mammals of the Tertiary included many offshoots from 
the main lines which we have traced. Among them were a number of 
genera of clumsy, ponderous brutes, some almost elephantine in their 
bulk. 

The carnivores. The ancestral lines of the families of the 
flesh eaters such as the cats (lions, tigers, etc.), the bears, the 
hyenas, and the dogs (including wolves and foxes) converge 
in the creodonts of the early Eocene, an order so generalized 
that it had affinities not only with the carnivores but also with 
the insect eaters, the marsupials, and the hoofed mammals as 
well. From these primitive flesh eaters, with small and simple 
brains, numerous small teeth, and plantigrade tread, the different 
families of the carnivores of the present have slowly evolved. 

Dogs and bears. The dog family diverged from the creodonts 
late in the Eocene, and divided into two branches, one of which 
evolved the wolves and the other the foxes. An offshoot gave 
rise to the family of the bears, and so closely do these two 
families, now wide apart, approach as we trace them back in 
Tertiary times that the Amphicyon, a genus doglike in its teeth 
and bearlike in other structures, is referred by some to the dog 
and by others to the bear family. The well-known plantigrade 
tread of bears is a primitive characteristic which has survived 
from their creodont ancestry. 

Cats. The family of the cats, the most highly specialized of all 
the carnivores, divided in the Tertiary into two main branches. 
One, the saber-tooth tigers (Fig. 351), which takes its name from 



414 THE ELEMENTS OF GEOLOGY 

their long, saberlike, sharp-edged upper canine teeth, evolved a 
succession of genera and species, among them some of the most 
destructive beasts of prey which ever scourged the earth. They 
were masters of the entire northern hemisphere during the 
middle Tertiary, but in Europe during the Pliocene they declined, 
from unknown causes, and gave place to the other branch of 
cats, which includes the lions, tigers, and leopards. In the 
Americas the saber-tooth tigers long survived the epoch. 




FIG. 351. Saber-Tooth Tiger 

Marine mammals. The carnivorous mammals of the sea - 
whales, seals, walruses, etc. seem to have been derived from 
some of the creodonts of the early Tertiary by adaptation to 
aquatic life. Whales evolved from some land ancestry at a very 
early date in the Tertiary ; in the marine deposits of the Eocene 
are found the bones of the Zeuglodon, a whalelike creature 
seventy feet in length. 

Primates. This order, which includes lemurs, monkeys, apes, 
and man, seems to have sprung from a creodont or insectivorous 
ancestry in the lower Eocene. Lemur-like types, with small, 
smooth brains, were abundant in the United States in the 
early Tertiary, but no primates have been found here in the mid- 
dle Tertiary and later strata. In Europe true monkeys were 



THE TERTIARY 415 

introduced in the Miocene, and were abundant until the close 
of the Tertiary, when they were driven from the continent by 
the increasing cold. 

Advance of the Mammalia during the Tertiary. During the 
several millions of years comprised in Tertiary tune the mam- 
mals evolved from the lowly, simple types which tenanted the 
earth at the beginning of the period, into the many kinds of 
highly specialized mammals of the Pleistocene and the present, 
each with the various structures of the body adapted to its own 
peculiar mode of life. The swift feet of the horse, the horns of 
cattle and the antlers of the deer, the lion's claws and teeth, 
the long incisors of the beaver, the proboscis of the elephant, 
were all developed in Tertiary times. In especial the brain of 
the Tertiary mammals constantly grew larger relatively to the 
size of body, and the higher portion of the brain the cerebral 
lobes increased in size in comparison with the cerebellum. 
Some of the hoofed mammals now have a brain eight or ten 
times the size of that of their early Tertiary predecessors of equal 
bulk. Nor can we doubt that along with the increasing size of 
brain went a corresponding increase in the keenness of the 
senses, in activity and vigor, and in intelligence. 



CHAPTEK XXII 
THE QUATERNARY 

The last period of geological history, the Quaternary, may be 
said to have begun when all, or nearly all, living species of 
mollusks and most of the existing mammals had appeared. 
It is divided into two great epochs. The first, the Pleistocene or 
Glacial epoch, is marked off from the Tertiary by the occupation 
of the northern parts of North America and Europe by vast ice 
sheets ; the second, the Recent epoch, began with the disappear- 
ance of the ice sheets from these continents, and merges into 
the present time. 

THE PLEISTOCENE EPOCH 

We now come to an episode of unusual interest, so different 
was it from most of the preceding epochs and from the present, 
and so largely has it influenced the conditions of man's life. 

The records of the Glacial epoch are so plain and full that 
we are compelled to believe what otherwise would seem almost 
incredible, that following the mild climate of the Tertiary 
came a succession of ages when ice fields, like that of Green- 
land, shrouded the northern parts of North America and Europe 
and extended far into temperate latitudes. 

The drift. Our studies of glaciers have prepared us to decipher 
and interpret the history of the Glacial epoch, as it is recorded 
in the surface deposits known as the drift. Over most of Canada 
and the northern states this familiar formation is exposed to 
view in nearly all cuttings which pass below the surface soil. 
The drift includes two distinct classes of deposits, the unstrati- 
fied drift laid down by glacier ice, and the stratified drift spread 
by glacier waters. 

416 



THE QUATERNARY 417 

The materials of the drift are in any given place in part unlike 
the rock on which it rests. They cannot be derived from the 
underlying rock by weathering, but have been brought from 
elsewhere. Thus where a region is underlain by sedimentary 
rocks, as is the drift-covered area from the Hudson Eiver to 
the Missouri, the drift contains not only fragments of limestone, 
sandstone, and shale of local derivation, but also pebbles of many 
igneous and metamorphic rocks, such as granites, gneisses, 




FIG. 352. Stratified Drift overlying Unstratified Drift, Massachusetts 

schists, dike rocks, quartzites, and the quartz of mineral veins, 
whose nearest source is the Archean area of Canada and the 
states of our northern border. The drift received its name 
when it was supposed that the formation had been drifted by 
floods and icebergs from outside sources, a theory long since 
abandoned. 

The distribution also of the drift points clearly to its peculiar origin. 
Within the limits of the glaciated area it covers the country without 
regard to the relief, mantling with its debris not only lowlands and 
valleys but also highlands and mountain slopes. 



418 THE ELEMENTS OF GEOLOGY 

The boundary of the drift is equally independent of the relief of the 
land, crossing hills and plains impartially, unlike water-laid deposits, 
whose margins, unless subsequently deformed, are horizontal. The 
boundary of the drift is strikingly lobate also, bending outward in 
broad, convex curves, where there are no natural barriers in the topog- 
raphy of the country to set it such a limit. Under these conditions such 
a lobate margin cannot belong to deposits of rivers, lakes, or ocean, 
but is precisely that which would mark the edge of a continental glacier 
which deployed in broad tongues of ice. 

The rock surface underlying the drift. Over much of its 
area the drift rests 011 firm, fresh rock, showing that both the 
preglacial mantle of residual waste and the partially decomposed 
and broken rock beneath it have been swept away. The under- 
lying rock, especially if massive, hard, and of a fine grain, has 
often been ground down to a smooth surface and rubbed to a 
polish as perfect as that seen on the rock beside an Alpine 
glacier where the ice has recently melted back. Frequently 
it has been worn to the smooth, rounded hummocks known 
as roches moutonnees, and even rocky hills have been thus 
smoothed to flowing outlines like roches moutonnees on a gigan- 
tic scale. The rock pavement beneath the drift is also marked 
by long, straight, parallel scorings, varying in size from deep 
grooves to fine striae as delicate as the hair lines cut by an en- 
graver's needle. Where the rock is soft or closely jointed it 
is often shattered to a depth of several feet beneath the drift, 
while stony clay has been thrust in among the fragments into 
which the rock is broken. 

In the presence of these glaciated surfaces we cannot doubt 
that the area of the drift has been overridden by vast sheets of 
ice which, in their steady flow, rasped and scored the rock bed 
beneath by means of the stones with which their basal layers 
were inset, and in places plucked and shattered it. 

Till. The unstratified portion of the drift consists chiefly of 
sheets of dense, stony clay called till, which clearly are the 



THE QUATERNARY 419 

ground moraines of ancient continental glaciers. Till is an 
unsorted mixture of materials of all sizes, from fine clay and 
sand, gravel, pebbles, and cobblestones, to large bowlders. The 
stones of the till are of many kinds, some having been plucked 
from the bed rock of the locality where they are found, and 
others having been brought from outside and often distant 
places. Land ice is the only agent known which can spread 
unstratified material in such extensive sheets. 

The fine material of the till comes from two different sources. 
In part it is derived from old residual clays, which in the 
making had been leached of the lime and other soluble ingredi- 
ents of the rock from which they weathered. In part it consists 
of sound rock ground fine ; a drop of acid on fresh, clayey till 
often proves by brisk effervescence that the till contains much 
undecayed limestone flour. The ice sheet, therefore, both scraped 
up the mantle of long-weathered waste which covered the coun- 
try before its coming, and also ground heavily upon the sound 
rock underneath, and crushed and wore to rock flour the 
fragments which it carried. 

The color of unweathered till depends on that of the materi- 
als of which it is composed. Where red sandstones have con- 
tributed largely to its making, as over the Triassic sandstones 
of the eastern states and the Algonkian sandstones about Lake 
Superior, the drift is reddish. When derived in part from coaly 
shales, as over many outcrops of the Pennsylvanian, it may 
when moist be almost black. Fresh till is normally a dull gray 
or bluish, so largely is it made up of the grindings of unoxidized 
rocks of these common colors. 

Except where composed chiefly of sand or coarser stuff, unweathered 
till is often exceedingly dense. Can you suggest by what means it has 
been thus compacted ? Did the ice fields of the Glacial epoch bear heavy 
surface moraines like the medial and lateral moraines of valley glaciers ? 
Where was the greater part of the load of these ice fields carried, judg- 
ing from what you know of the glaciers of Greenland ? 



420 THE ELEMENTS OF GEOLOGY 

Bowlders of the drift. The pebbles and bowlders of the drift 
are in part stream gravels, bowlders of weathering, and other 
coarse rock waste picked up from the surface of the country by 
the advancing ice, and in part are fragments plucked from 
ledges of sound rock after the mantle of waste had been 
removed. Many of the stones of the till are dressed as only 
glacier ice can do ; their sharp edges have been blunted and 
their sides faceted and scored. 

We may easily find all stages of this process represented among the 
pebbles of the till. Some are little worn, even on their edges ; some 
are planed and scored on one side only ; while some in their long jour- 
ney have been ground down to many facets and have lost much of their 
original bulk. Evidently the ice played fast and loose with a stone 

carried in its basal layers, 
now holding it fast and 
rubbing it against the rock 
beneath, now loosening its 
grasp and allowing the 
stone to turn. 

Bowlders of the drift 

are sometimes found on 
IG. 353. A Drumlm, Wisconsin 

higher ground than their 

parent ledges. Thus bowlders have been left on the sides of Mount 
Katahdin, Maine, which were plucked from limestone ledges twelve miles 
distant and three thousand feet lower than their resting place. In other 
cases stones have been carried over mountain ranges, as in Vermont, 
where pebbles of Burlington red sandstone were dragged over the Green 
Mountains, three thousand feet in height, and left in the Connecticut 
valley sixty miles away. No other geological agent than glacier ice 
could do this work. 

The bowlders of the drift are often large. Bowlders ten and twenty 
feet in diameter are not uncommon, and some are known whose diam- 
eter exceeds fifty feet. As a rule the average size of bowlders decreases 
with increasing distance from their sources. Why ? 

Till plains. The surface of the drift, where left in its initial 
state, also displays clear proof of its glacial origin. Over large 




THE QUATERNARY 



421 



areas it is spread in level plains of till, perhaps bowlder-dotted, 
similar to the plains of stony clay left in Spitzbergen by the 
recent retreat of some of the glaciers of that island. In places 




FIG. 354. Map of a Portion of a Dramlin Area near Oswego, New York 

the unstratified drift is heaped in hills of various kinds, which 
we will now describe. 

Drumlins. Drumlins are smooth, rounded hills composed of 
till, elliptical in base, and having their longer axes parallel to 
the movement of the ice as shown by glacial scorings. They 



422 THE ELEMENTS OF GEOLOGY 

crowd certain districts in central New York and in southern 
Wisconsin, where they may be counted by the thousands. 
Among the numerous drumlins about Boston is historic Bunker 
Hill. 

Drumlins are made of ground moraine. They were accumu- 
lated and given shape beneath the overriding ice, much as are 
sand bars in a river, or in some instances were carved, like 
roches moutonnees, by an ice sheet out of the till left by an 
earlier ice invasion. 

Terminal moraines. The glaciated area is crossed by belts of 
thickened drift, often a mile or two, and sometimes even ten 




Fi<;. 355. Terminal Moraine, Staten Island 

miles and more, in breadth, which lie transverse to the move- 
ment of the ice and clearly are the terminal- moraines of ancient 
ice sheets, marking either the limit of their farthest advance or 
pauses in their general retreat. 

The surface of these moraines is a jumble of elevations and 
depressions, which vary from low, gentle swells and shallow 
sags to sharp hills, a hundred feet or so in height, and deep, 
steep-sided hollows. Such tumultuous hills and hummocks, set 



THE QUATERNARY 428 

with depressions of all shapes, which usually are without outlet 
and are often occupied by marshes, ponds, and lakes, surely 
cannot be the work of running water. The hills are heaps of 
drift, lodged beneath the ice edge or piled along its front. The 
basins were left among the tangle of morainic knolls and ridges 
(Fig. 105) as the margin of the ice moved back and forth. 
Some bowl-shaped basins were made by the melting of a mass of 
ice left behind by the retreating glacier and buried in its debris. 

The stratified drift. Like modern glaciers the ice sheets of 
the Pleistocene were ever being converted into water about their 
margins. Their limits on the land were the lines where their 
onward flow was just bal- 
anced by melting and evap- 
oration. On the surface of 
the ice along the marginal 
zone, rivulets no doubt 
flowed in summer, and found 
their way through crevasses 
to the interior of the 
glacier or to the ground. 
Subglacial streams, like 
those of the Malaspina FIG. 356. Esker, New York 

glacier, issued from tunnels in the ice, and water ran along the 
melting ice front as it is seen to do about the glacier tongues 
of Greenland. All these glacier waters flowed away down the 
chief drainage channels in swollen rivers loaded with glacial 
waste. 

It is not unexpected therefore that there are found, over all 
the country where the melting ice retreated, deposits made of 
the same materials as the till, but sorted and stratified by run- 
ning water. Some of these were deposited behind the ice front 
in ice-walled channels, some at the edge of the glaciers by issu- 
ing streams, and others were spread to long distances in front 
of the ice edge by glacial waters as they flowed away. 




424 



THE ELEMENTS OF GEOLOGY 



Eskers are narrow, winding ridges of stratified sand and 
gravel whose general course lies parallel with the movement of 
the glacier. These ridges, though evidently laid by running 
water, do not follow lines of continuous descent, but may be 




FIG. 357. Kames, New York 

found to cross river valleys and ascend their sides. Hence the 
streams by which eskers were laid did not flow unconfmed upon 
the surface of the ground. We may infer that eskers were 
deposited in the tunnels and ice-walled gorges of glacial streams 
before they issued from the ice front. 

Kames are sand and gravel knolls, associated for the most 
part with terminal moraines, and heaped by glacial waters along 

the margin of the ice. 

Kame terraces are hum- 

rnocky embankments of 

stratified drift sometimes 
FIG. 358. Diagram illustrating the For- found in rugged regions 

along the sides of valleys. 

In these valleys long 




mation of Kame Terraces 
i, glacier ice ; t, t, terraces 



tongues of glacier ice lay slowly melting. Glacial waters took 
their way between the edges of the glaciers and the hillside, 
and here deposited sand and gravel in rude terraces. 



THE QUATERNARY 425 

Outwash plains are plains of sand and gravel which frequently 
border terminal moraines on their outward face, and were spread 
evidently by outwash from the melting ice. Outwash plains are 
sometimes pitted by bowl-shaped basins where ice blocks were 
left buried in the sand by the retreating glacier. 

Valley trains are deposits of stratified drift with which river 
valleys have been aggraded. Valleys leading outward from the 
ice front were flooded by glacial waters and were filled often to 
great depths with trains of stream-swept drift. Since the disap- 
pearance of the ice these glacial flood plains have been dissected 
by the shrunken rivers of recent times and left on either side the 
valley in high terraces. Valley trains head in morainic plains, 
and their material grows finer down valley and coarser toward 
their sources. Their gradient is commonly greater than that of 
the present rivers. 

The extent of the drift. The extent of the drift of North 
America and its southern limits are best seen in Figure 359. Its 
area is reckoned at about four million square miles. The ice 
fields which once covered so much of our continent were all 
together ten times as large as the inland ice of Greenland, and 
about equal to the enormous ice cap which now covers the 
antartic regions. 

The ice field of Europe was much smaller, measuring about 
seven hundred and seventy thousand square miles. 

Centers of dispersion. The direction of the movement of the 
ice is recorded plainly in the scorings of the rock surface, in 
the shapes of glaciated hills, in the axes of drumlius and eskers, 
and in trains of bowlders, when the ledges from which they 
were plucked can be discovered. In these ways it has been 
proved that in North America there were three centers where 
ice gathered to the greatest depth, and from which it flowed 
in all directions outward. There were thus three vast ice 
fields, one the Cordilleran, which lay upon the Cordilleras of 
British America ; one the Keewatin, which flowed out from the 



426 



THE ELEMENTS OF GEOLOGY 



province of Keewatin, west of Hudson Bay ; and one the Labrador 
ice field, whose center of dispersion was on the highlands of the 
peninsula of Labrador. As shown in Figure 359, the western ice 
field extended but a short way beyond the eastern foothills of 
the Rocky Mountains, where perhaps it met the far-traveled ice 




FIG. 359. Hypothetical Map of the Pleistocene Ice Sheets of North America 
From Salisbury's Glacial Geology of New Jersey 

from the great central field. The Keewatin and the Labrador 
ice fields flowed farthest toward the south, and in the Mississippi 
valley the one reached the mouth of the Missouri and the other 
nearly to the mouth of the Ohio. In Minnesota and Wisconsin 
and northward they merged in one vast field. 



THE QUATERNARY 



427 



The thickness of the ice was so great that it buried the high- 
est mountains of eastern North America, as is proved by the 
transported bowlders which have been found upon their sum- 
mits. If the land then stood at its present height above sea 
level, and if the average slope of the ice were no more than ten 
feet to the mile, a slope so gentle that the eye could not 
detect it and less than half the slope of the interior of the 
inland ice of Green- 
land, the ice pla- 
teaus about Hudson 
Bay must have 
reached a thickness 
of at least ten 
thousand feet. 



In Europe the 
Scandinavian plateau 
was the chief center 
of dispersion. At the 
time of greatest glaci- 




FIG. 360. Hypothetical Map of the Pleistocene 
Ice Sheet of Europe 



ation a continuous field of ice extended from the Ural Mountains to the 
Atlantic, where, off the coasts of Norway and the British Isles, it met 
the sea in an unbroken ice wall. On the south it reached to southern 
England, Belgium, and central Germany, and deployed on the eastern 
plains in wide lobes over Poland and central Russia (Fig. 360). 

At the same time the Alps supported giant glaciers many times the 
size of the surviving glaciers of to-day, and a piedmont glacier covered 
Lhe plains of northern Switzerland. 

The thickness of the drift. The drift is far from uniform hi 
thickness. It is comparatively thin and scanty over the Lauren- 
tian highlands and the rugged regions of New England, while 
from southern New York and Ontario westward over the Mis- 
sissippi valley, and on the great western plains of Canada, it 
exceeds an average of one hundred feet over wide areas, and in 
places has five and six times that thickness. It was to this 



428 THE ELEMENTS OF GEOLOGY 

marginal belt that the ice sheets brought their loads, while 
northwards, nearer the centers of dispersion, erosion was exces- 
sive and deposition slight. 

Successive ice invasions and their drift sheets. Recent stud- 
ies of the drift prove that it does not consist of one indivis- 
ible formation, but includes a number of distinct drift sheets, 
each with its own peculiar features. The Pleistocene epoch 
consisted, therefore, of several glacial stages, during each of 
which the ice advanced far southward, together with the 
intervening interglacial stages when, under a milder climate, 
the ice melted back toward its sources or wholly disappeared. 

The evidences of such interglacial stages, and the means by which 
the different drift sheets are told apart, are illustrated in Figure 361. 
Here the country from N to S is wholly covered by drift, but the drift 




FIG. 861. Diagram illustrating Criteria by which Different 
Drift Sheets are distinguished 

from N to m is so unlike that from m to S that we may believe it the 
product of a distinct ice invasion and deposited during another and far 
later glacial stage. The former drift is very young, for its drainage is 
as yet immature, and there are many lakes and marshes upon its sur- 
face ; the latter is far older, for its surface has been thoroughly dissected 
by its streams. The former is but slightly weathered, while the latter 
is so old that it is deeply reddened by oxidation and is leached of its 
soluble ingredients such as lime. The younger drift is bordered by a 
distinct terminal moraine, while the margin of the older drift is not 
thus marked. Moreover, the two drift sheets are somewhat unlike in 
composition, and the different proportion of pebbles of the various 
kinds of rocks which they contain shows that their respective glaciers 
followed different tracks and gathered their loads from different regions. 
Again, in places beneath the younger drift there is found the buried 
land surface of an older drift with old soils, forest grounds, and vege- 
table deposits, containing the remains of animals and plants, which 
tell of the climate of the interglacial stage in which they lived. 



THE QUATERNARY 429 

By such differences as these the following drift sheets have 
been made out in America, and similar subdivisions have been 
recognized in Europe. 

5 The Wisconsin formation 
4 The lowan formation 
3 The Illinoian formation 
2 The Kansan formation 
1 The Xebraskan formation 

The earliest glacial stage is recorded in an exceedingly old 
drift sheet found in eastern Nebraska and other states and 
parted from the overlying Kansan by the deposits of the first 
interglacial epoch, called the Aftonian. Quite generally pre- 
Kansan drift has been swept away by later ice invasions, but 
in places it is found deeply buried beneath their ground moraines. 

The two succeeding stages mark the greatest snowfall of the 
Glacial epoch. In Kansan times the Keewatin ice field slowly 
grew southward until it reached fifteen hundred miles from its 
center of dispersion and extended from the Arctic Ocean to 
northeastern Kansas. In the Illinoian stage the Labrador ice 
field stretched from Hudson Straits nearly to the Ohio Eiver in 
Illinois. In the lowan and the Wisconsin, the closing stages of 
the Glacial epoch, the readvancing ice fields fell far short of their 
former limits in the Mississippi valley, but in the eastern states 
the Labrador ice field during Wisconsin times overrode for the 
most part all earlier deposits, and, covering New England, prob- 
ably met the ocean in a continuous wall of ice which set its bergs 
afloat from Massachusetts to northern Labrador. 

We select for detailed description the Kansan and the Wis- 
consin formations as representatives, the one of the older and 
the other of the younger drift sheets; 

The Kansan formation. The Kansan drift consists for the 
most part of a sheet of clayey till carrying smaller bowlders 
than the later drift. Few traces of drumlins, kames, or terminal 




430 



THE QUATERNARY 



431 



moraines are found upon the Kansan drift, and where thick 
enough to mask the preexisting surface, it seems to have been 
spread originally in level plains of tilL 

The initial Kansan plain has been worn by running water 
until there are now left only isolated patches and the narrow 
strips and crests of the divides, which still rise to the ancient 
leveL The valleys of the larger streams have been opened wide. 
Their well-developed tributaries have carved nearly the entire 
plain to valley slopes (Figs. 50 B, and 59 ). The lakes and marshes 
which once marked the infancy of the region have long since 




FIG. 363. Plain of Wisconsin Drift, Iowa 

been effaced The drift is also deeply weathered. The till, origi- 
nally blue in color, has been yellowed by oxidation to a depth 
of ten and twenty feet and even more, and its surface is some- 
times rusted to terra-cotta red. To a somewhat less depth it has 
been leached of its lime and other soluble ingredients. In the 
weathered zone its pebbles, especially where the till is loose in 
texture, are sometimes so rotted that granites may be crumbled 
with the fingers. The Kansan drift is therefore old. 

The Wisconsin formation. The Wisconsin drift sheet is but 
little weathered and eroded, and therefore is extremely young. 
Oxidation has effected it but slightly, and lime and other 



432 THE ELEMENTS OF GEOLOGY 

soluble plant foods remain undissolved even at the grass roots. 
Its river systems are still in their infancy (Fig. 50, A). Swamps 
and peat bogs are abundant on its undrained surface, and to 
this drift sheet belong the lake lands of our northern states 
and of the Laurentian peneplain of Canada. 

The lake basins of the Wisconsin drift are of several different 
classes. Many are shallow sags in the ground moraine. Still more 
numerous are the lakes set in hollows among the hills of the terminal 
moraines ; such as the thousands of lakelets of eastern Massachusetts. 
Indeed, the terminal moraines of the Wisconsin drift may often be 
roughly traced on maps by means of belts of lakes and ponds. Some 
lakes are due to the blockade of ancient valleys by morainic debris, 
and this class includes many of the lakes of the Adirondacks, the 
mountain regions of New England, and the Laurentian area. Still 
other lakes rest in rock basins scooped out by glaciers. In many cases 
lakes are due to more than one cause, as where preglacial valleys have 
both been basined by the ice and blockaded by its moraines. The Finger 
lakes of New York, for example, occupy, such glacial troughs. 

Massive terminal moraines, which mark the farthest limits to 
which the Wisconsin ice advanced, have been traced from Cape 
Cod and the islands south of New England, across the Appala- 
chians and the Mississippi valley, through the Dakotas, and far 
to the north over the plains of British America. Where the ice 
halted for a time in its general retreat, it left recessional mo- 
raines, as this variety of the terminal moraine is called. The 
moraines of the Wisconsin drift lie upon the country like great 
festoons, each series of concentric loops marking the utmost 
advance of broad lobes of the ice margin and the various pauses 
in their recession. 

Behind the terminal moraines lie wide till plains, in places 
studded thickly with drumlins, or ridged with an occasional 
esker. Great outwash plains of sand and gravel lie in front of 
the moraine belts, and long valley trains of coarse gravels tell 
of the swift and powerful rivers of the time. 



THE QUATERNARY 



433 



The loess of the Mississippi valley. A yellow earth, quite 
like the loess of China, is laid broadly as a surface deposit over 
the Mississippi valley from eastern Nebraska to Ohio outside 
the boundaries of the lowan and the Wisconsin drift. Much 
of the loess was deposited in lowan times. It is younger than 
the earlier drift sheets, for it overlies their weathered and eroded 
surfaces. It thickens to the lowan drift border, but is not found 
upon that drift. It is older than the Wisconsin, for in many 
places it passes underneath the Wisconsin terminal moraines. 




FIG. 364. Bank of Loess, Iowa 

In part the loess seems to have been washed from glacial waste 
and spread in sluggish glacial waters, and in part to have been 
distributed by the wind from plains of aggrading glacial streams. 
The effects of the ice invasions on rivers. The repeated ice 
invasions of the Pleistocene profoundly disarranged the drainage 
systems of our northern states. In some regions the ancient 
valleys were completely filled with drift. On the withdrawal 
of the ice the streams were compelled to find their way, as best 
they could, over a fresh land surface, where we now find them 
flowing on the drift in young, narrow channels. But hundreds of 



434 



THE ELEMENTS OF GEOLOGY 



feet below the ground the well driller and the prospector for coal 
and oil discover deep, wide, buried valleys cut in rock, the 
channels of preglacial and interglacial streams. In places the 
ancient valleys were filled with drift to a depth of a hundred 

feet, and sometimes even to 
a depth of four hundred and 
five hundred feet. In such 
valleys, rivers now flow high 
above their ancient beds of 
rock on floors of valley drift. 
Many of the valleys of our 
present rivers are but patch- 
works of preglacial, inter- 
glacial, and postglacial 
courses (Fig. &66). Here 
the river winds along an 
ancient valley with gently 
sloping sides and a wide 
alluvial floor perhaps a mile 
or so in width, and there it 
enters a young, rock-walled 
gorge, whose rocky bed may 
be crossed by ledges over 
which the river plunges in 
waterfalls and rapids. 

In such cases it is possible 
that the river was pushed to 
one side of its former valley 
by a lobe of ice, and com- 
pelled to cut a new channel in the adjacent uplands. A section 
of the valley may have been blockaded with morainic waste, 
and the lake formed behind the barrier may have found outlet 
over the country to one side of the ancient drift-filled valley. 
In some instances it would seem that during the waning of the 



N.Mai'WslIe~ ( fSw.VA.~j ^ MARYLAND 




FIG. 365. Preglacial Drainage, Upper 

Ohio Valley 
After Chamberlin and Leverett 




THE QUATERNARY 435 

ice sheets, glacial streams, while confined within walls of stag- 
nant ice, cut down through the ice and incised their channels 
on the underlying country, in some cases being let down on old 
river courses, and in other cases excavating gorges in adjacent 
uplands. 

Pleistocene lakes. Temporary lakes were formed wherever 
the ice front dammed the natural drainage of the region. Some, 
held in the minor valleys 
crossed by ice lobes, were 
small, and no doubt many 
were too short-lived to leave 
lasting records. Others, X v ~~~~~ 

long held against the north- 
ward sloping country by the FIG. 366. A Patchwork Valley 
retreating ice edge, left in a and a', ancient courses still occupied by 
their beaches, their clayey the river ; 6 po^glacial gorge ; c, ancient 
J J course now filled with drift 

beds, and their outlet chan- 
nels permanent evidences of their area and depth. Some of 
these glacial lakes are thus known to have been larger than any 
present lake. 

Lake Agassiz, named in honor of the author of the theory of conti- 
nental glaciation, is supposed to have been held by the united front of 
the Keewatin and the Labrador ice fields as they finally retreated down 
the valley of the Red River of the North and the drainage basin of 
Lake Winnipeg. From first to last Lake Agassiz covered a hundred and 
ten thousand square miles in Manitoba and the adjacent parts of Min- 
nesota and North Dakota, an area larger than all the Great Lakes 
combined. It discharged its waters across the divide which held it on 
the south, and thus excavated the valley of the Minnesota River. The 
lake bed a plain of till was spread smooth and level as a floor with 
lacustrine silts. Since Lake Agassiz vanished with the melting back 
of the ice beyond the outlet by the Nelson River into Hudson Bay, 
there has gathered on its floor a deep humus, rich in the nitrogenous 
elements so needful for the growth of plants, and it is to this soil that 
the region owes its well-known fertility. 



436 THE ELEMENTS OF GEOLOGY 

The Great Lakes. The basins of the Great Lakes are broad 
preglacial river valleys, warped by movements of the crust still 
in progress, enlarged by the erosive action of lobes of the con- 
tinental ice sheets, and blockaded by their drift. The compli cated 
glacial and postglacial history of the lakes is recorded in old 
strand lines which have been traced at various heights about 
them, showing their areas and the levels at which their waters 
stood at different times. 

With the retreat of the lobate Wisconsin ice sheet toward 
the north and east, the southern and western ends of the basins 
of the Great Lakes were uncovered first ; and here, between the 
receding ice front and the slopes of land which faced it, lakes 
gathered which increased constantly in size. 

The lake which thus came to occupy the western end of the 
Lake Superior basin discharged over the divide at Duluth down 
the St. Croix River, as an old outlet channel proves ; that which 
held the southern end of the basin of Lake Michigan sent its 
overflow across the divide at Chicago via the Illinois River to 
the Mississippi ; the lake which covered the lowlands about the 
western end of Lake Erie discharged its waters at Fort Wayne 
into the Wabash River. 

The ice still blocked the Mohawk and St. Lawrence valleys 
on the east, while on the west it had retreated far to the north. 
The lakes become confluent in wide expanses of water, whose 
depths and margins, as shown by their old lake beaches, varied 
at different times with the position of the confining ice and 
with warpings of the land. These vast water bodies, which at 
one or more periods were greater than all the Great Lakes com- 
bined, discharged at various times across the divide at Chicago, 
near Syracuse, New York, down the Mohawk valley, and by a 
channel from Georgian Bay into the Ottawa River. Last of all 
the present outlet by the St. Lawrence was established. 

The beaches of the glacial lakes just mentioned are now far from 
horizontal. That of the lake which occupied the Ontario basin has an 



THE QUATERNARY 437 

elevation of three hundred and sixty-two feet above tide at the west and 
of six hundred and seventy-five feet at the northeast, proving here a 
differential movement of the land since glacial times amounting to 
more than three hundred feet. The beaches which mark the successive 
heights of these glacial lakes are not parallel ; hence the warping began 
before the Glacial epoch closed. We have already seen that the canting 
of the region is still in progress (p. 198). 

The Champlain subsidence. As the Glacial epoch approached 
its end, and the Labrador ice field melted back for the last time 
to near its source, the land on which the ice had lain in eastern 
North America was so depressed that the sea now spread far and 
wide up the St. Lawrence valley. It joined with Lake Ontario, 
and extending down the Champlain and Hudson valleys, made 
an island of New England and the maritime provinces of Canada. 

The proofs of this subsidence are found in old sea beaches 
and sea-laid clays resting on Wisconsin till. At Montreal such 
terraces are found six hundred and twenty feet above sea level, 
and along Lake Champlain where the skeleton of a whale 
was once found among them at from five hundred to four 
hundred feet. The heavy delta which the Mohawk Eiver built 
at its mouth in this arm of the sea now stands something more 
than three hundred feet above sea level. The clays of the Cham- 
plain subsidence pass under water near the mouth of the Hud- 
son, and in northern New Jersey they occur two hundred feet 
below tide. In these elevations we have measures of the warp- 
ing of the region since glacial times. 

The western United States in glacial times. The western 
United States was not covered during the Pleistocene by any 
general ice sheet, but all the high ranges were capped with 
permanent snow and nourished valley glaciers, often many times 
the size of the existing glaciers of the Alps. In almost every 
valley of the Sierras and the Eockies the records of these van- 
ished ice streams may be found in cirques, glacial troughs, roches 
moutonne*es, and morainic deposits. 



438 THE ELEMENTS OF GEOLOGY 

It was during the Glacial epoch that Lakes Bonneville and 
Lahontan (p. 107) were established in the Great Basin, whose 
climate must then have been much more moist than now. 

The driftless area. In the upper Mississippi valley there is an area 
of about ten thousand square miles in southwestern Wisconsin and the 
adjacent parts of Iowa and Minnesota, which escaped the ice invasions. 
The rocks are covered with residual clays, the product of long pre- 
glacial weathering. The region is an ancient peneplain, uplifted and 
dissected in late Tertiary times, with mature valleys whose gentle 




FIG. 367. A Valley in the Driftless Area 

gradients are unbroken by waterfalls and rapids. Thus the driftless 
area is in strong contrast with the immature drift topography about it, 
where lakes and waterfalls are common. It is a bit of preglacial land- 
scape, showing the condition of the entire region before the Glacial 
epoch. 

The driftless area lay to one side of the main track of both the Kee- 
watin and the Labrador ice fields, and at the north it was protected by 
the upland south of Lake Superior, which weakened and retarded the 
movement of the ice. 

South of the driftless area the Mississippi valley was invaded at dif- 
ferent times by ice sheets from the west, the Kansan and the lowan, 
and again by the Illinoian ice sheet from the east. Again and again the 



THE QUATERNARY 



439 



Mississippi River was pushed to one side or the other of its path. The 
ancient channel which it held along the Illinoian ice front has been 
traced through southeastern Iowa for many miles. 

Benefits of glaciation. Like the driftless area, the preglacial 
surface over which the ice advanced seems to have been well 
dissected after the late Tertiary uplifts, and to have been carved 
in many places to steep valley slopes and rugged hills. The 
retreating ice sheets, which left smooth plains and gently rolling 
country over the wide belt where glacial deposition exceeded 




FIG. 368. Cross Section of a Valley in Eastern Iowa 

a, country rock ; b, Kansan till ; c, loess ; t, terrace of reddish sands and decayed 
pebbles above reach of present stream ; s, stream ; fp, flood plain of s. What 
is the age of rock-cut valley and of the alluvium which partially fills it, com- 
pared with that of the Kansan till ? with that of the loess ? Give the complete 
history recorded in the section. 

glacial erosion, have made travel and transportation easier than 
they otherwise would have been. 

The preglacial subsoils were residual clays and sands, com- 
posed of the insoluble elements of the country rock of the local- 
ity, with some minglings of its soluble parts still undissolved. 
The glacial subsoils are made of rocks of many kinds, still un- 
decayed and largely ground to powder. They thus contain an 
inexhaustible store of the mineral foods of plants, and in a form 
made easily ready for plant use. 

On the preglacial hillsides the humus layer must have been 
comparatively thin, while the broad glacial plains have gathered 



440 THE ELEMENTS OF GEOLOGY 

deep black soils, rich in carbon and nitrogen taken from the 
atmosphere. To these soils and subsoils a large part of the 
wealth and prosperity of the glaciated regions of our country 
must be attributed. 

The ice invasions have also added very largely to the water 
power of the country. The rivers which in preglacial times 
were flowing over graded courses for the most part, were pushed 
from their old valleys and set to flow on higher levels, where 
they have developed waterfalls and rapids. This power will 
probably be fully utilized long before the coal beds of the coun- 
try are exhausted, and will become one of the chief sources of 
the national wealth. 

The Recent epoch. The deposits laid since glacial times 
graduate into those now forming along the ocean shores, on 
lake beds, and in river valleys. Slow and comparatively slight 
changes, such as the warpings of the region of the Great Lakes, 
have brought about the geographical conditions of the present. 
The physical history of the Eecent epoch needs here no special 
mention. 

THE LIFE OF THE QUATERNARY 

During the entire Quaternary, invertebrates and plants suf- 
fered little change in species, so slowly are these ancient and 
comparatively simple organisms modified. The Mammalia, on the 
other hand, have changed much since the beginning of Quater- 
nary time : the various species of the present have been evolved, 
and some lines have become extinct. These highly organized 
vertebrates are evidently less stable than are lower types of ani- 
mals, and respond more rapidly to changes in the environment. 

Pleistocene mammals. In the Pleistocene the Mammalia 
reached their culmination both in size and in variety of forms, 
and were superior in both these respects to the mammals of 
to-day. In Pleistocene times in North America there were sev- 
eral species of bison, one whose widespreading horns were 



THE QUATERNARY 



441 



ten feet from tip to tip, a gigantic moose elk, a giant rodent 
(Castoroides) five feet long, several species of musk oxen, 
several species of horses, more akin, however, to zebras than 




FIG. 369. Megatherium 

to the modern horse, a huge lion, several saber-tooth tigers, 
immense edentates of several genera, and largest of all the 
mastodon and mammoth. 

The largest of the edentates was the Megatherium, a clumsy ground 
sloth bigger than a rhinoceros. The bones of the Megatherium are 
extraordinarily massive, the thigh bone being thrice as thick as that 
of an elephant," and the animal seems to have been well able to get its 




FIG. 370. Glyptodon 

living by overthrowing trees and stripping off their leaves. The Glyp- 
todon was a mailed edentate, eight feet long, resembling the little arma- 
dillo! These edentates survived from Tertiary times, and in the warmer 
stages of the Pleistocene ranged north as far as Ohio and Oregon. 



442 



THE ELEMENTS OF GEOLOGY 



The great proboscidians of the Glacial epoch were about the 
size of modern elephants, and somewhat smaller than their 
ancestral species in the Pliocene. The Mastodon ranged over all 
North America south of Hudson Bay, but had become extinct 
in the Old World at the end of the Tertiary. The elephants 
were represented by the Mammoth, which roamed in immense 
herds from our middle states to Alaska, and from Arctic Asia 
to the Mediterranean and Atlantic. 

It is an oft-told story how about a century ago, near the Lena 
River in Siberia, there was found the body of a mammoth which had 
been safely preserved in ice for thousands of years, how the flesh was 
eaten by dogs and bears, and how the eyes and hoofs and portions 
of the hide were taken with the skeleton to St. Petersburg. Since 
then several other carcasses of the mammoth, similarly preserved in 
ice, have been found in the same region, one as recently as 1901. 
We know from these remains that the animal was clothed in a coat of 
long, coarse hair, with thick brown fur beneath. 


The distribution of animals and plants. The distribution of 

species in the Glacial epoch was far different from that of the 

present. In the glacial 
stages arctic species 
ranged south into what 
are now temperate lati- 
tudes. The walrus 
throve along the shores 
of Virginia and the 
musk ox grazed in Iowa 
and Kentucky. In 

FIG. 371. Skull of Musk Ox, from Pleisto- Europe the reindeer and 
cene Deposits, Iowa arcfcic fox reacned the 

Pyrenees. During the Champlain depression arctic shells lived 
along the shore of the arm of the sea which covered the St. 
Lawrence valley. In interglacial times of milder climate the 
arctic fauna-flora retreated, and their places were taken by plants 




THE QUATERNARY 443 

and animals from the south. Peccaries, now found in Texas, 
ranged into Michigan and New York, while great sloths from 
South America reached the middle states. Interglacial beds at 
Toronto, Canada, contain remains of forests of maple, elm, and 
papaw, with mollusks now living in the Mississippi basin. 

What changes in the forests of your region Would be brought about, 
and in what way, if the climate should very gradually grow colder? 
What changes if it should grow warmer ? 

On the Alps and the highest summits of the White Mountains of 
New England are found colonies of arctic species of plants and insects. 
How did they come to be thus separated from their home beyond the 
arctic circle by a thousand miles and more of temperate climate 
impossible to cross? 

Man. Along with the remains of the characteristic animals 
of the time which are now extinct there have been found in 
deposits of the Glacial epoch in the Old World relics of Pleisto- 
cene Man, his bones, and articles of his manufacture. In Europe, 
where they have best been studied, human relics occur chiefly 
in peat bogs, in loess, in caverns where man made his home, 
and in high river terraces sometimes eighty and a hundred feet 
above the present flood plains of the streams. 

In order to understand the development of early man 5 we 
should know that prehistoric peoples are ranked according to 
the materials of which their tools were made and the skill 
shown in their manufacture. There are thus five well-marked 
stages of human culture preceding the written annals of history. 

The Eolithic stage (the Dawn of the Stone Age) is attested 
only by chipped flints, so crudely worked that there has been 
much doubt whether they may not have been produced by 
natural processes, such as frost, the impact of pebbles in swift 
torrents, and the pressure of overlying beds. 

This stage may have been preceded by an earlier, in which 
man's only tools and weapons were sticks of wood, rock splinters, 
and chance pebbles of brook or beach. 



444 



THE ELEMENTS OF GEOLOGY 



In the Paleolithic stage (the Old Stone Age) the first imple- 
ments were nodules of flint chipped rudely to sharp edges and 
held unhafted in the hand as a general-purpose tool (Fig. 372). 
With the progress of thousands of years these handstones were 
much improved, and flint flakes were retouched to a variety of 

characteristic forms .adapted 
to many different uses. 

The Neolithic stage (the 
New Stone Age) witnessed 
the introduction of weapons 
and tools of polished stone. 
To this stage belonged the 
North American Indian at 
the time of the discovery of 
the continent. 

The Bronze stage and the 
Iron stage were marked by 
the introduction of these 
metals. 

Pithecanthropus (ape man) 
erectus. The oldest remains 

of man are parts of a skeleton found in Java in river deposits 
late Pliocene or very early Pleistocene in age. Their antiquity 
is therefore to be measured in hundreds of thousands of years. 
These parts comprise (1) the skullcap, with a retreating fore- 
head as low as a chimpanzee's, and with beetling brow ridges 
of ferocious aspect ; (2) three molar teeth somewhat coarser 
than man's to-day ; and (3) a thigh bone whose articulations 
indicate an erect posture and whose length proves a stature 
about equal to that of the average Anglo-Saxon. The cranial 
capacity, estimated at about 900 cubic centimeters, is well 
above the ape's (which never exceeds 600 cubic centimeters), 
although it is much less than that of the lowest of existing 
human races. The estimated ratio of brain to body weight 




FIG. 372. Paleolithic Implement from 
Great Britain 



THE QUATERNARY 445 

(1 : 94) is also intermediate between that of the ape (1 : 183) 
and that of man (1:51). The low and narrow frontal area 
of the brain, the seat of the higher mental faculties, the control 
of conduct, and the registration of experience, indicates that the 
Javau ape man was a creature of feeble intelligence and little 
wisdom. The brain area of motor speech is twice that of the 
highest ape, although but one-half that of present man, and 
thus permits the inference that Pithecanthropus may have pos- 
sessed the rudiments of language. Certainly he had attained 
a stage far higher than any of the primates below man. 
Arboreal life had been abandoned ; erect posture freed the fore 
limbs from the burden of the support of the body and from the 
work of locomotion, and gave the advantage of agile movements 
of defense and offense ; and the large increase of brain was no 
doubt accompanied by a higher mentality, able to discern rela- 
tions which no ape can grasp. 

The ancestors of Javan man are not to be found in any 
existing apes (the structure of man proves this), but common 
ancestral species may be expected far back in Tertiary times, 
although such have not yet been found. Man may not have 
been exempt from evolution (that method of creation which 
has made all life on earth akin), and inherited apelike char- 
acters of teeth, jaw, skull, and other parts persisted long in 
early species of mankind. But the line separating the human 
from the brute was passed when a quickened intelligence was 
able to discern the relation of means to ends, to reason, to 
conceive abstract ideas, and to feel moral obligation. In the 
advent of man, as in the beginning of life upon the planet, 
creative energy rose to a higher plane; and the vast possi- 
bilities of self-directed progress now opened up in man can 
well be compared with the progress which life had already 
achieved during the geologic ages. 

Heidelberg man. The next glimpse obtained of man's long 
upward path is after a lapse of scores of thousands of years, 




446 THE ELEMENTS OF GEOLOGY 

wlien another race emerges in western Europe during the 
second interglacial epoch, that following the Kansan glaciation. 
Of this forerunner only a lower jaw has been discovered, in place 
beneath seventy -nine feet of loess and glacial sands near Heidel- 
berg. This jaw is the largest and most massive of all known 
jaws of man. It bears no trace of a chin prominence and in 
various other characteristics is distinctly apelike. But the well- 
preserved teeth are wholly human. Associated with the jaw 

were found Eolithic imple- 
^^^ ments and the bones of an 

* . , : f J extinct elephant and rhi- 

noceros and the cave bear 
an( i ]j on animals which 
then roamed widely over 
southern Europe. Heidel- 
berg man was probably the 
FIG. 373. Jaw of Heidelberg Man ancestor of the race next 

mentioned. 

Neanderthal man. The skeletal remains of this peculiar 
species have been found in more than twenty places in west- 
ern Europe. Neanderthal man was short, averaging about five feet 
four inches in height, with broad, stooping shoulders, a short, 
thick neck, and an enormous head hung forward because of the 
curve of the spinal column. The magnum 'foramen, the open- 
ing admitting the spinal cord to the skull, was placed, apelike, 
somewhat in the rear of its central position in the skull of 
man to-day. The knees were permanently bent, and short shin 
bones show a plodding gait. As with Heidelberg man, his jaw 
was chinless. As with Pithecanthropus, overhanging brow ridges 
were parted from a low, retreating forehead by a shallow groove. 
During the Wisconsin glaciation the Neanderthal ers found 
shelter in the caverns and under the overhanging cliffs of the 
rivers of western Europe. In these cave deposits, and some- 
times beneath scores of feet of cave earth, river sands, and 






447 



448 



THE ELEMENTS OF GEOLOGY 




stalagmitic crusts, are found not only their fossil skeletal 
remains but also their tools and weapons, along with the 
bones of various animals on which they fed. Thus we know 
that Neanderthal man was a hunting savage. He had learned 
the art of kindling fire. He flaked flints to forms suitable for 
scrapers, drills, knives, and spearheads. He interred his dead 
with ceremony. A Neanderthal youth of about sixteen years, 
for example, was found in a cave in France carefully laid to 

rest upon a pillow of worked 
flints. The body had been 
extended to full length, the 
face toward the rising sun. 
The head rested on the bent 
right arm. At the left hand 
was placed a masterpiece of 
the weapons of the tribe. 
Charred bones of joints of 
beef had been laid about. 
These primitive men felt 
pitying love and sorrow and had faith in life beyond the grave. 
The Neanderthal, Heidelberg, and Java races, with others' as 
primitive, of which space does not permit description, show 
that the human stock early branched into different species and 
genera of sub-men, which long since vanished from the earth. 
Cro-Magnon man. With the close of the Wisconsin glaciation 
the slow, squat, stupid Neanderthalers were replaced by a race 
of the finest physique and high intelligence. The Cro-Magnons, 
as these immigrants are called, were one of the tallest races of 
mankind. Long shin bones show them swift of foot. The pre- 
frontal area of their large brain was well developed, and face 
and jaw were fully human. 

The Cro-Magnons carried well-nigh to perfection the art of 
flaking flint, but they used bone more largely. Of this tough 
and easily worked substance they made needles, which imply 



FIG. 375. Cro-Magnon Drawing of 
a Reindeer 



THE QUATERNARY 



449 



sewed clothing, tallies, spoons, whistles, and barbed harpoons 
for spearing salmon. Carved wands, used perhaps as insignia 
of office, suggest a tribal organization. The love of personal 
adornment is seen in armlets and necklaces of shells. 

To the Cro-Magnons belongs the credit of the world's first 
art in drawing, sculpture, and painting. They etched on plaques 
of bone and ivory, modeled in clay, 
sculptured in stone or bone, and 
frescoed on the walls of their caves 
the animals about them, the cave 
bear and lion, the mammoth and 
woolly rhinoceros, the wild horse, 
the reindeer, boar, and bison. The 
vigor, freshness, and sure touch seen 
in hundreds of their works of art 
are unapproached among untaught 
peoples. But as their art galleries 
were often placed in remote, dark 
chambers of their caverns, it may 
be supposed that these frescoes were 
somehow connected with magic rites 
employed to secure success in hunt- 
ing. With all his intelligence, Cro- 
Magnon man was still a hunting 
savage. He knew no agriculture, 




FIG. 376. Head of Girl, Cro- 
Magnon Sculpture in Ivory 
(Enlarged) 

From Brassempouy. After Piette, 
L'anthr., 6, PI. 6, 1895 



made no pottery, wove no cloth, and domesticated no animals, 
not even man's first and best friend, the dog. 

The Recent epoch. The Glacial epoch ends with the melting 
of the ice sheets of North America and Europe and the replace- 
ment of the Pleistocene mammalian fauna by present species. 
How gradually the one epoch shades into the other is seen in 
the fact that the glaciers which still linger in Norway and 
Alaska are the lineal descendants or the renewed appearances 
of the ice fields of Glacial times. 



450 THE ELEMENTS OF GEOLOGY 

Neolithic man. The wild Paleolithic men vanished from 
Europe with the wild beasts which they hunted, and their 
place w T as taken by tribes, perhaps from Asia, of a higher 
culture. The remains of Neolithic man are found, much as 
are those of the North American Indians, upon or near the 
surface, in burial mounds, in shell heaps (the refuse heaps of 
their settlements), in peat bogs, caves, recent flood-plain deposits, 
and in the beds of lakes near shore, where they sometimes built 
their dwellings upon piles. 

The successive stages in European culture are well displayed 
in the peat bogs of Denmark. The lowest layers contain the pol- 
ished stone implements of Xeolithic man, along with remains of 
the Scotch fir. Above are oak trunks with implements of bronze, 
while the higher layers hold iron weapons and the remains of a 
beech forest. 

Neolithic man in Europe had learned to make pottery, to 
spin and weave linen, to hew timbers and build boats, and to 
grow wheat and barley. He had domesticated the dog, horse, 
sheep, goat, hog, and two varieties of cattle neither of which 
is known to have reached Europe before this time. He set up 
huge stones, singly or in lines or circles, and built rude tombs 
of stone, often covered with great mounds of earth. Man had 
not yet learned the art of extracting metals from their ores. 

In North America no proof has been found that any race 
ever entered the continent before the coining of the immediate 
ancestors of the American Indian from Asia, an event which 
seems to have taken place after the close of the Glacial epoch. 

The Neolithic stage of culture passed into the age of bronze 
and that into the age of iron at different times with different 
peoples. Civilization began with permanent settlements, as 
along the lower Tigris and Euphrates and the Nile. And with 
the introduction of the art of writing, history, based on written 
records, takes up the tale. 



THE QUATERNARY 451 

Our brief study of the outlines of geology has given us, it is 
hoped, some great and lasting good. To conceive a past so differ- 
ent from the present has stimulated the imagination, and to 
follow the inferences by which the conclusions of our science 
have been reached has exercised one of the noblest faculties of 
the mind, the reason. We have learned to look on nature in 
new ways : every landscape, every pebble, now has a meaning 
and tells something of its origin and history, while plants and 
animals have a closer interest since we have traced the long 
lines of their descent. The narrow horizons of human life have 
been broken through, and we have caught glimpses of that 
immeasurable reach of time in which nebulae and suns and 
planets run their courses. Moreover, we have learned some- 
thing of that orderly and world-embracing progress by which 
the once uninhabitable globe has come to be man's well- 
appointed home, and life appearing in the lowliest forms has 
steadily developed higher and still higher types. Seeing this 
process enter human history and lift our race continually to 
loftier levels, we find reason to believe that the onward, up- 
ward movement recorded hi the geological past is the mani- 
festation of the same wise Power which makes for righteousness 
and good, and that this unceasing purpose will still lead 011 to 
nobler ends. 



INDEX 



Aa, lava, 241 

Acadian coal field, 354 

Accretion hypothesis, 304 

Acidic rocks, 249 

Adelsberg grotto, 47 

Adirondacks, 309, 816 

Africa, 357 

Agassiz, Lake, 67, 111, 435 

Agates, 251 

Alabama, 317, 360 

Alaska, 85, 138, 140, 378 

Aletsch glacier, 121 

Algae, 51, 52 

Algonlyan group, 306, 310 

Allegheny Mountains, 90, 224, 

403 

Alluvial cones, 98 
Alluvium, 62 
Alps, 118, 121, 141, 210, 211, 

223, 229, 349, 427, 443 
Amazon River, 175 
Ammonites, 294, 367, 380, 382 
Amphibians, 364, 383 
Amphicyon, 413 
Amygdules, 250 
Andes, 236, 279 
Angle of repose, 25 
Antarctic continent, 294 
Antecedent streams, 209 
Antelope, 413 
Anthracite, 281 
Anticlinal folds, 203, 209 
Ants, 20 

Apennine Mountains, 399 
Appalachia, 317, 351, 358 



Appalachian coal field, 356 

Appalachian deformation, 358 

Appalachian Mountains, 211, 214, 
218, 292 

Aquifer, 44 

Aragonite, 296 

Archseopteryx, 393 

Archeozoic era, 306 

Arenaceous rocks, 9 

Argillaceous rocks, 9 

Arizona, 32, 75, 145, 151, 154, 220, 
229, 249, 257, 371, 390 

Arkansas, 337, 356, 373 

Arkose, 186, 282, 370 
326, Artesian wells, 44 

Arthropods, 322 

Artiodactyls, 411 

Assiniboine, Mount, 34 
212, Atlas Mountains, 399 

Atmosphere, 304, 305 

Atolls, 191, 193 

Augite, 274 

Austin, Tex., 71 

Australia, 190, 357 

Avalanches, 26 

Bad Lands, 397, 398 
Baltic Sea, 170, 171, 199 
Barite, 287 

Barrier Reefs, 191, 192 
Basal conglomerate, 173, 184 
Basalt, 249 
Baselevel, 80, 83 
Basic rocks, 249 
Basin deposits, 103 
453 



454 



THE ELEMENTS OF GEOLOGY 



Bay bars, 164 

Beaches, 162, 164 

Bears, 413 

Bedding planes, 5 

Belemnites, 382 

Belt Mountains, 309 

Bergschrund, 121, 135, 137 

Bermudas, 148 

Birds, 392 

Bison, 413 

Bitter Boot Mountains, 272 

Black Hills, 309, 371 

Blastoids, 339 

Blastosphere, 311 

Block mountains, 222, 226 

Blowholes, 159 

Blue Ridge, 309, 316 

Bomb, volcanic, 256 

Bonneville, Lake, 107, 198, 438 

Bosses, 270 

Bowlders, erratic, 420 

of weathering, 28 

Brachiopods, 323, 333, 343, 364, 380 
Brazil, 18, 236 
Breccia, 218, 255, 264 
British Columbia, 373, 378 
Bronze stage, 443, 448 
Bryozoans, 333 
Bunker Hill, 422 

Calamites, 361, 367 
Calcareous rocks, 9 
Calciferous series, 327 
Calcite, 296 
Caldera, 239 

California, 24, 99, 136, 152, 158, 159, 
170, 197, 224, 256, 262, 287, 357, 
360, 371, 400 

Great Valley of, 101, 199, 372, 396 
Cambrian period, 315 

glaciation in, 358 

life of, 319 



Camels, 412 

Canada, 28, 35, 67, 69, 90, 182, 198, 

200, 213, 218, 267, 307, 309, 316, 

336, 354, 357, 432, 437 
Cape Breton Island, 198 
Cape Cod, 152 
Carbonated springs, 261 
Carbonates, "formation of, 12 
Carboniferous period, 350 

life of, 361 
Carnivores, 413 
Cascade Mountains, 90, 400 
Cats, 413 

Catskill Mountains, 342 
Caucasus Mountains, 399 
Caverns, 45, 241 
Cenozoic era, 394 
Centipedes, 333 
Cephalopods, 324, 333, 339, 344, 367, 

380 

Ceratites, 380 
Ceratosaurus, 385 
Chain coral, 339, 343 
Chalcopyrite, 287 
Chalk, 9, 374, 375 
Chalybeate springs, 52 
Champlain subsidence, 437 
Charleston earthquake, 233 
Chazy series, 327 
Chelan, Lake, 141 
Chemung series, 341, 342 
Chesapeake Bay, 169, 170, 197 
Chicago, 146, 198, 436 
Chile, 235 
China, 28, 151 
Christmas Island, 194, 248 
Cincinnati anticline, 329, 356 
Cirques, 135 
Clinton series, 335 
Coal, 352, 370, 375 
Coal Measures, 351 
Coast Range, 101, 372, 399 



INDEX 



455 



Coastal plain, Atlantic, 183 

Ccelenterates, 320 

Coke, 271 

Colorado, 18, 29, 33, 37, 153, 233, 

266, 271, 334 

Colorado plateaus, 357, 403 
Colorado River, 30, 75, 140, 154, 223, 

307, 313, 317 
Columbia lavas, 400 
Columnar structure, 253 
Concretions, 49 
Cones, alluvial, 98 

volcanic, 257 
Conglomerate, 9, 173 
Congo River, 175 
Conifers, 377 

Connecticut valley, 370, 403 
Contemporaneous lava sheets, 248, 

268 

Continental delta, 175, 183 
Continental shelf, 183 
Continents, 188 
Contours, 69 
Copper, 287, 310 
Coquina, 177 
Coral reefs, 188 

Corals, ancient, 321, 332, 338, 379 
Cordaites, 363 
Cordilleran ice field, 425 
Corniferous series, 341 
Coves, 161 
Crabs, 379 

Crandall volcano, 263, 400 
Crater Lake, 259 
Creodonts, 413 
Cretaceous period, 372 
Crinoids, 332, 363, 379 
Crocodiles, 384 
Cro-Magnon man, 448 
Cross bedding, 65, 182 
Crustacea, 322, 332, 363, 379 
Crustal movements, 195 



Cumberland plateau, 90 

Cup corals, 338 

Cycads, 377, 378 

Cycle of erosion, 84, 185, 292 

Cystoids, 321, 332, 367 

Dalmatia, 170 

Darwin's theory of coral reefs, 191 

Dead Sea, 221, 279 

Death Gulch, 264 

Deep-sea deposits, 187 

Deer, 413 

Deflation, 152 

Deformation, 279 

Delaware River, 197, 403 

Deltas, 108, 111, 197 

of Ganges, 109 

of Indus, 110 

of Mississippi, 109, 197 
Denudation, 57 
Denver, 398 
Desert, 15, 55 
Devitrification, 257 
Devonian period, 316, 341 
Dicotyls, 377, 404 
Digitigrade, 406 
Dikes, 244, 265 
Dinosaurs, 385 
Dinothere, 410 
Diorite, 274 
Dip, 202 
Dip fault, 225 
Diplodocus, 286 
Dipnoans, 346 
Discina, 324 
Dismal Swamp, 106 
Dogs, 413 
Dragon flies, 364 
Drift, 18, 113, 416 

bowlders of, 420 

englacial, 125 

extent of, 426 



456 



THE ELEMENTS OF GEOLOGY 



Drift, pebbles of, 114, 420 

stratified, 423 

thickness of, 429 
Driftless area, 438 
Drowned valleys, 197 
Drumlins, 421 
Duluth, 436 
Dunes, 147 
Dust falls, 145 

Earth, age of, 292, 298, 302 

interior of, 276 
Earthquakes, 224, 233 

causes of, 233, 237 

Charleston, 233 

distribution of, 236 

geological effects of, 234 

India, 235 

Japan, 237 

New Madrid, 236 
Earthworms, 20, 21 
Echinoderms, 321, 332, 333, 343, 

363 

Edentates, 441 
Egypt, 93 
Electric Peak, 269 
Elephants, 410 
Elevation, effects of, 85 

' movements of, 197 
Eocene epoch, 395 
Epicontinental seas, 318 
Erratics, 133, 420 
Eskers, 424 
Etna, 248, 402 
Europe, Pleistocene ice sheet of, 

427 

Eurypterids, 333, 339, 363, 367 
Evolution, 300, 447 

Faceted pebbles, 113, 114, 420 
Falls of the Ohio, 343 
Fan folds, 205 



Fault scarps, 219 

Faults, 217 

Faunas, 299 

Feldspar, 9, 10, 42 

Ferns, 361 

Finger lakes, 432 

Fire clay, 353 

Fishes, 334, 339, 345, 364, 405 

Fissure eruptions, 242 

Fissure springs, 44 

Fjords, 139, 142 

Flint, 18, 375 

Flood plains, 85, 93 

Floods, 54 

Floras, 299 

Florida, 46, 163, 177, 178, 188, 396 

Flow lines, 252 

Fluorite, 287 

Folded mountains, 210 

Folds, 201, 208 

Foliation, 283 

Foraminifera, 187, 374 

Forests, Carboniferous, 354, 361 

Cretaceous, 377, 378 

Devonian, 343 

Tertiary, 404 
Fort Wayne, 436 
Fossils, 177, 296 
Fractures, 215 
Fragmental rocks, 8 
France, 167, 171 ; cave men of, 445 
Fringing reefs, 190 
Frogs, 383 
Frost, 15 

Funafuti atoll, 194 
Fundy, Bay of, 182 

Gabbro, 274 
Ganges, 58, 109, 197 
Ganoids, 347 
Garnet, 281 
Gases, volcanic, 244 



INDEX 



457 



Gastropods, 324 

Gastrula, 311 

Geneva, Lake, 71 

Geodes, 49 

Geological time, divisions of, 296 

Geology, definition of, 1, 3 

departments of, 4 
Georgia, 18, 373 
Geysers, 52, 260 
Glacial epoch, 142, 416 
Glaciers, 113 

abrasion by, 133 

Alpine, 118 

compared with rivers, 137, 142 

crevasses of, 123 

deposition by, 138 

Greenland, 116 

lower limit of, 129 

melting of, 126 

mode of formation, 118 

moraines, 124 

motion of, 120, 122, 134 

piedmont, 131, 141 

plucking by, 133 

tables, 130 

transportation by, 132 

troughs, 137 

wells, 129 

young and mature, 129 
Glauconite, 176 
Globigerina ooze, 187 
Giyptodon, 441 
Gneiss, 283 
Goats, 413 
Gold, 287, 372 
Goniatite, 344, 367 
Graded slopes, 25 
Granite, 9, 274 
Graphite, 312 
Graptolites, 320, 339 
Gravitation, 22 
Great Basin, 867, 360, 374, 376 



Great Lakes, 198, 436 
Great Plains, 82 
Great Salt Lake, 107 
Greenland, 115, 126, 378 
Green Mountains, 309, 316, 420 
Green sand, 176 
Ground water, 39 
Ground water surface, 40 
Gryphsea, 379 
Gymnosperms, 363, 377 
Gypsum, 12, 335, 357, 371 

Hade, 217 

Hamilton series, 341 

Hanging valley, 139 

Hanging wall, 217 

Hartz Mountains, 214 

Hawaiian volcanoes, 238, 248, 258, 

279 

Heat and cold, 13 
Heidelberg man, 445 
Helderberg series, 341 
Hematite, 310 
Henry Mountains, 271, 375 
High Plains, 100, 398 
Killers Mountain, 271 
Himalayas, 122, 209, 210, 399 
^Historical geology, 4, 291 
Honeycomb corals, 339 
Hood, Mount, 260, 262 
Hooks, 165 
Hornblende, 274 
Hornblende schist, 284 
Hudson Bay, 90, 170 
Hudson River, 197, 493 
Hudson series, 327, 329 
Humus acids, 10 
Humus layer, 19 
Huronian systems, 308 
Hwang-ho River, 151 
Hydrosphere, 22 
Hydrozoa, 320 



458 



THE ELEMENTS OF GEOLOGY 



Icebergs, 116, 143 

Iceland, 242, 258 

Ichthyosaurus, 389 

Idaho, 34, 400 - 

Igneous rocks, 9, 249, 250, 251, 278 

Illinoian formation, 429 

Illinois, 54, 146, 356, 374 

India, 28, 102, 147, 235, 357, 402 

Indian Territory, 356 

Indiana, 48, 104 

Indo-gangetic plain, 101 

Indus River, 101, 110 

Insects, 333, 364, 380 

Interior of earth, 276 

Internal geological agencies, 195 

Intrusive masses, 270 

Intrusive rocks, 273 

Intrusive sheets, 268 * 

Inverness earthquake, 236 

Iowa, 29, 69, 73, 80, 86, 336, 356, 

374, 431, 433, 439, 442 
lowan formation, 429 
Iron ores, 13, 53, 279, 310 
Islands, coral, 188 
wave cut, 159, 161 

Japan, 223, 224, 237 
Joints, 5, 31, 216 
Jordan valley, 279 
Jura Mountains, 141, 212 
Jurassic period, 369 

Kame terraces, 424 

Kames, 424 

Kansan formation, 429 

Kansas, 41, 50, 100, 336, 357, 373, 

374, 429 
Kaolin, 12 
Karst, 47, 198 
Katahdin, Mount, 420 
Keewatin ice field, 425 
Kentucky, 45, 46, 343, 442 



Keweenawan system, 308, 310 
Kilauea, 239 

Kings River Canyon, 403 
Krakatoa, 246 

Labrador, 198 

Labrador ice field, 426 

Laccolith, 271 

Lagoon, 165, 167 

Lahontan, Lake, 107, 438 

Lake Chelan, 141 

Lake dwellings, 448 

Lake Geneva,' 71 

Lake Superior region, 284, 308, 310 

Lakes, 70, 222, 432 

basins, 97, 110, 127, 139, 141, 164, 
165, 167, 191, 221, 222, 235, 259, 
423, 432, 435 

deposits, 104 

glacial, 127, 139, 141, 423, 432, 435 

Pleistocene, 435 

salt, 106 
Laminae, 5 
Landslides, 26, 234 
Lapilli, 255 
Laramie series, 375 
Laurentian peneplain, 84, 308, 432 
Lava, 238, 241 
Lava domes, 243, 400 
Lepidodenclron, 362, 367 
Lichens, 16 
Lignite, 271 

Limestone, 7, 177, 178, 190 
Limonite, 13 
Lingulella, 324 
Lithosphere, 21 
Lizards, 384 
Llamas, 412 
Loess, 150, 433 
Long Island, 373 
Louisiana, 336, 396 
Lower Silurian period, 327 



INDEX 



459 



Luray Cavern, 48 
Lycopods, 362 

Magnetite, 279, 310 

Maine, 169, 420 

Malaspina glacier, 131 

Maldive Archipelago, 193 

Mammals, 393, 405, 440 

Mammoth, 442 

Mammoth Cave, 46 

Mammoth Hot Springs, 52 

Man, 414, 443 

Mantle of waste, 17 

Marble, 284, 329 

Marengo Cavern, 48 

Marl, 104 

Marsupials, 393, 406 

Martha's Vineyard," 161, 373, 396 

Maryland, 56, 270 

Massachusetts, 106, 162, 267, 309, 

403, 417, 429 
Mastodon, 410, 441, 442 
Matterhorn, 34 
Maturity of land forms, 80 
Mauna Loa, 239 
Meanders, 96 
Medina series, 335, 403 
Megatherium, 441 
Mendota, Lake, 71 
Mesa, 31, 32, 153 
Mesozoic era, 369 
Mesozoic peneplain, 376, 403 
Metamorphism, 281 
Mexico, 373, 375 
Mica, 9 

Mica schist, 284 
Michigan, 104, 356, 443 
Michigan, Lake, 149, 198 
Mineral veins, 49, 286 
Minnesota, 97, 426 
Miocene series, 395 
Mississippi, 337 



Mississippi embayment, 373, 374, 396 
Mississippi River, 66, 67, 82, 94, 96, 

109 

Mississippian series, 350 
Missouri, 18, 236 
Missouri River, 56, 97 
Mobile Bay, 197 
Mohawk valley, 436, 437 
Molluscous shell deposits, 177 
Mollusks, 324 
Monadnock, 83 
Monkeys, 414 
Monoclinal fold, 204 
Monocotyls, 377, 404 
Monotremes, 393, 406 
Montana, 71, 309, 313, 373 
Montreal, 268, 437 
Monuments, 33 
Moraines, 124 
Mosasaurs, 390 
Mountain sheep, 413 
Mountains, age of, 229 

life history of, 212, 215 

origin of, 90, 210, 222 

sculpture of, 33, 137 
Movements of crust, 195 
Muir glacier, 122, 129 

Nantucket, 373 

Naples, 201 

Narragansett Bay, 197 

Natural bridges, 46 

Natural gas, 330 

Natural levees, 93 

Nautilus, 334 

Neanderthal man, 446 

Nebraska, 50, 82, 100, 255, 356 

Nebular hypothesis, 304 

Neolithic man, 444, 450 

Nevada, 1C4, 107, 222, 288, 289, 360, 

400 
Ne"ve", 120 



460 



THE ELEMENTS OF GEOLOGY 



New England, 88, 373, 376, 378, 395, 

403, 429, 432, 437 
Newfoundland, 198 
New Jersey, 148, 166, 168, 176, 196, 

268, 269, 309, 310, 373, 437 
New Madrid earthquake, 236 
New Mexico, 31, 371, 399 
New York, 60, 90, 309, 327, 329, 335, 

336, 360, 421, 422, 423, 424, 432, 

443 

Niagara Falls, 60, 199 
Niagara series, 335 
Nile, 93, 109, 197 
Normal fault, 217 
North Carolina, 106 
North Dakota, 67 
North Sea, 170 
Notochord, 347 
Nova Scotia, 198 
Nunatak, 116, 132 

Ohio, 82, 198, 329, 335, 441 

Ohio Kiver, 55, 82 

Oil, 330 

Olenellus zone, 323 

Olivine, 274 

Oolitic limestone, 178 

Ooze, deep-sea, 131 

Ordovician period, 316, 327 

life of, 331 

Oregon, 222, 262, 400 
Oreodon, 412 
Ores, 287, 290 
Organisms, work of, 16 
Oriskany series, 341 
Ornithostoma, 392 
Orthoceras, 325, 367, 380 
Oscillations, 196 

a cause of, 273 

effect on drainage, 85 
Ostracoderms, 344 
Ottawa River, 90 



Outcrop, 2 
Outliers, 31 
Outwash plains, 425 
Oxidation, 13 
Oyster, 379, 380 

Pahoehoe lava, 241 
Palseospondylus, 344 
Paleolithic man, 444 
Paleozoic era, 315 
Palisades of Hudson, 268 
Palms, 377 
Pamir, 15 
Peat, 94, 104 
Peccaries, 412 
Pelecypods, 324 
Pele"e, Mt., 246 
Peneplain, 83 

dissected, 86 

Laurentian, 89, 308, 402 

Mesozoic, 376, 403 
Pennsylvania, 211, 257, 357, 359, 403 
Pennsylvanian series, 350, 351 
Perissodactyl, 408 
Perlitic structure, 252 
Permian series, 350, 357, 360, 366 
Petrifaction, 296 
Petroleum, 330, 343 
Phenacodus, 406 
Phyllite, 283 
Phyllopod, 323 

Piedmont Belt, 87, 214, 309, 374 
Piedmont plains, 99 
Pikes Peak, 18 

Pithecanthropus erectus, 444-5 
Placers, 287 

Plains of marine abrasion, 172 
Planation, 81 

Planetesiinal hypothesis, 304 
Plantigrade, 406 
Platte River, 82 
Playa, 103 



INDEX 



461 



Playa lakes, 104 
Pleistocene epoch, 416 
Plesiosaurus, 389, 390 
Pliocene epoch, 395 
Plucking, 133 
Po River, 68, 197 
Pocono sandstone, 360, 404 
Porosity of rocks, 40 
Porphyritic structure, 262 
Potholes, 59 

Potomac River, 68, 66, 403 
Predentata, 386 
Primates, 414 

Prince Edward Island, 198 
Proboscidians, 410, 441, 442 
Proterozoic era, 306 
Pteropods, 325 
Pterosaurs, 391 
Puget Sound, 396 
Pumice, 260 
Pyrite, 13 

Quarry water, 15 
Quartz, 6, 9 
Quartz schist, 284 
Quaternary period, 395, 416 
Quebec, 28 

Rain, erosion, 23 

Rain prints, 181 

Recent epoch, 416, 440, 449 

Re concentration of ores, 289 

Record, the geological, 291 

Red clay, 187 

Red River of the l?orth, 67 

Red Sea, 221 

Red snow, 115 

Reefs, coral, 188 

Regional intrusions, 272 

Reptiles, 367, 383 

Rhinoceros, 408 

Rhizocarp, 343 

Rhode Island, 356 



Rhone glacier, 123 
Rhyolite, 249 
Richmond, Va., 370 
Rift valleys, 221 
Ripple marks, 180 
Rivers, 64 

bars, 66 

braided channels, 94 

deltas, 108 

deposition, 62 

discharge, 66 

erosion, 69 

estuaries, 86 

flood plains, 93 

floods, 64 

graded, 74 

gradients, 82 

load of, 66 

mature, 72, 80, 97, 98 

meanders, 96 

plains, 99 

profile of, 73 

revived, 86 

run-off, 54 

structure of deposits, 102 

terraces, 96 

transportation, 66, 64 

waterfalls, 78 

young, 67 

Roches moutonne'es, 134, 418 
Rock bench, 156 
Rock salt, 12, 357, 371 
Rocky Mountains, 376, 399, 437 
Ruminants, 412 

Saber-tooth tiger, 413 
Saguenay River, 90, 201 
Sahara, 15, 146, 150 
St. Elias Range, 399 
St. Peter sandstone, 160 
Salamanders, 383 
Salina series, 335 



462 



THE ELEMENTS OF GEOLOGY 



Salt, common, 106, 335 
Salt lakes, 106 
San Francisco Bay, 197 
Sand, beach, 163 

of deserts, 149 

reefs, 165, 167 

storms, 145 
Sandstone, 6, 7, 186 
Sarcoui, 258 
Sauropoda, 386 
Schist, 283 
Schladebach, 277 
Scoria, 250, 255 
Scorpions, 339, 340, 363 
Scotland, 170, 220, 402 
Sea, 155 

erosion, 156 

deposition, 174 

transportation, 162 
Sea arch, 159 
Sea cave, 158 
Sea cliff, 156, 157 
Sea cucumber, 363 
Seals, 414 
Sea stacks, 169 
Sea urchin, 332, 379 
Seaweed, 176 
Sedimentary rocks, 8, 9 
Selkirk Mountains, 218 
Septa, 338 
Sequoia, 378 
Shale, 8, 9 
Sharks, 345, 405 
Shasta, Mount, 262, 400 
Sheep, 413 

Shenandoah valley, 403 
Shores of elevation, 167 
Shores of depression, 169 
Siderite, 53 

Sierra Nevada Mountains, 24, 90, 99, 
224, 229, 272, 287, 318, 357, 371, 
372, 396, 398, 399, 402, 437 



Sigillaria, 362, 367 

Silica, 6, 178 

Silurian period, 316, 334 

life of, 338 
Sink hole, 46 
Slate, 207, 282 
Slaty cleavage, 207 
Slickensides, 217 
Snake River lavas, 400, 401 
Snakes, 384, 405 
Soil, 19 
Solfatara, 260 
Solution, 11 
Soufriere, 246 
South America, 357 
South Carolina, 233 
South Dakota, 276, 374, 397 
Spanish Peaks, 271, 376 
Spherulites, 252 
Spiders, 363 
Spitzbergen, 378 
Sponges, 320, 379 
Springs, 41 

thermal, 50 
Stalactite, 48 
Stalagmite, 48 
Starfishes, 332 
Staubbach, 140 
Stegosauras, 387 
Stoss side, 134 
Stratification, 5, 64, 180 
Strise, glacial, 114, 133, 418 
Strike, 203 
Strike fault, 225 
Stromatopora, 331, 379 
Stromboli, 244 
Subsidence, 85, 183, 197 
Sun cracks, 180 
Superior, Lake, 257 
Superposition, law of, 293 
Susquehanna River, 403 
Sutlej River, 209 



INDEX 



463 



Sweden, 199 
Swine, 412 
Switzerland, 28, 427 
Syenite, 274 
Synclinal fold, 204 
Syracuse, N.Y., 436 
Syringopora, 339 

Tabulae, 339 

Taconic deformation, 329 

Taconic Mountains, 376 

Talc, 284 

Talc schist, 284 

Talus, 23 

Tapir, 409 

Teleost fishes, 349, 382, 405 

Tennessee, 90, 373 

Terminal moraines, 126, 422, 432 

Terraces, 86, 96 

Tertiary period, 395 

Texas, 15, 69, 71, 166, 336, 356, 357, 

371, 373, 374, 378 
Theromorphs, 383 
Throw, 217 
Thrust faults, 217 
Till, 418 ; till plains, 420 
Toronto, 443 
Trachyte, 249, 258 
Travertine, 52 
Trenton series, 327 
Triassic period, 369 
Triceratops, 387 

Trilobites, 322, 332, 339, 363, 367 
Tuff, 255 
Turkestan, 103 
Turtles, 384 
Tyrannosaur, 386 

Unconformity, 227 

Undertow, 174 

Utah, 107, 271, 360, 371, 398, 399 

Utica series, 327 



V-Valleys, 74 

Valley drift, 128 

Valley trains, 425 

Valleys, 66 

Vermont, 309, 329, 420 

Vernagt glacier, 129 

Vertebrates, 334, 349 

Vesuvius, 247, 259, 402 

Virginia, 48, 84, 106, 370, 403, 442 

Volcanic ashes, 244, 255 

cones, 257 

necks, 267 

rocks, 249 
Volcanoes, 238 

causes of, 278 

decadent, 260 

submarine, 248 

tertiary, 399 

Walrus, 414 . 
Warped valleys, 101 
Warping, 198 
Wasatch Mountains, 375 
Washington, 18, 91, 150, 400 
Waterfalls, 59, 78 
Waves, 156 
Weathering, 5 

chemical, 10 

differential, 29 

mechanical, 13 
Wells, 41 

artesian, 44 

West Virginia, 79, 357, 359 
White Mountains, 443 
Wind, 144 

deposition, 147 

erosion, 151 

pebbles carved by, 152 

transportation, 145 
Wisconsin, 15, 18, 70, 71, 90, 94, 422, 

426 

Wisconsin formation, 429, 431 
Wyoming, 50, 98, 371 



464 INDEX 

Yahtse River, 131 Zeuglodon, 414 

Yellow Sea, 151, 170 Zone of cementation, 49, 180 

Yellowstone canyon, 74 Zone of solution, 45 

Yellowstone National Park, 50, 51, Zones of flow and of fracture, 207 

52, 260, 261, 263, 269, 400 
Yosemite, 403 



ANNOUNCEMENTS 



THE ELEMENTS OF GEOLOGY 

By WILLIAM HARMON NORTON 

Alumni Professor of Geology in Cornell College, Mt. Vernon, Iowa 



8vo, cloth, 461 pages, illustrated 



THE essentials of the science of geology are treated 
with fullness and ample illustration in this text-book 
for beginners. By limiting his discussion chiefly to this 
continent the author has been able to devote a large 
amount of space to the principles which he describes. The 
following characteristics are important. 

1. The outline is exceptionally simple. Under the leading geological 
processes are grouped the rock structures and land forms of which 
they are the cause. 

2. The inductive method is emphasized throughout. Concrete 
examples are given large space as the basis of generalizations of the 
science. Numerous exercises and problems, many of which are in the 
form of diagrams, are designed to train the pupil and to test his 
knowledge. 

3. The cycle idea is made prominent, and both the records of erosion 
and those of sedimentation are given special attention. 

4. In historical geology a broad view is afforded of the development 
of the North American continent and of the evolution of life upon the 
earth. Only the leading types of plants and animals are mentioned, 
and special attention is given to those which mark the lines of descent 
of forms now living. 

The book is designed for use in high schools and acad- 
emies, and may also be found useful in short elementary 
college courses. 



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